Improve magic functions docs (#391)

* Improve documentation of magic functions

* Improve wording for decorators

include/mg_procedure.h

@@ -1559,7 +1559,7 @@ enum mgp_error mgp_func_add_opt_arg(struct mgp_func *func, const char *name, str
 typedef void (*mgp_func_cb)(struct mgp_list *, struct mgp_func_context *, struct mgp_func_result *,
                             struct mgp_memory *);
 
-/// Register a Memgraph magic function
+/// Register a Memgraph magic function.
 ///
 /// The `name` must be a sequence of digits, underscores, lowercase and
 /// uppercase Latin letters. The name must begin with a non-digit character.

include/mgp.py

@@ -1179,16 +1179,15 @@ def read_proc(func: typing.Callable[..., Record]):
     """
     Register `func` as a read-only procedure of the current module.
 
-    `read_proc` is meant to be used as a decorator function to register module
-    procedures. The registered `func` needs to be a callable which optionally
-    takes `ProcCtx` as the first argument. Other arguments of `func` will be
-    bound to values passed in the cypherQuery. The full signature of `func`
-    needs to be annotated with types. The return type must be
-    `Record(field_name=type, ...)` and the procedure must produce either a
-    complete Record or None. To mark a field as deprecated, use
-    `Record(field_name=Deprecated(type), ...)`. Multiple records can be
-    produced by returning an iterable of them. Registering generator functions
-    is currently not supported.
+    The decorator `read_proc` is meant to be used to register module procedures.
+    The registered `func` needs to be a callable which optionally takes
+    `ProcCtx` as its first argument. Other arguments of `func` will be bound to
+    values passed in the cypherQuery. The full signature of `func` needs to be
+    annotated with types. The return type must be `Record(field_name=type, ...)`
+    and the procedure must produce either a complete Record or None. To mark a
+    field as deprecated, use `Record(field_name=Deprecated(type), ...)`.
+    Multiple records can be produced by returning an iterable of them.
+    Registering generator functions is currently not supported.
 
     Example usage.
 
@@ -1222,16 +1221,16 @@ def write_proc(func: typing.Callable[..., Record]):
     """
     Register `func` as a writeable procedure of the current module.
 
-    `write_proc` is meant to be used as a decorator function to register module
+    The decorator `write_proc` is meant to be used to register module
     procedures. The registered `func` needs to be a callable which optionally
     takes `ProcCtx` as the first argument. Other arguments of `func` will be
     bound to values passed in the cypherQuery. The full signature of `func`
     needs to be annotated with types. The return type must be
     `Record(field_name=type, ...)` and the procedure must produce either a
     complete Record or None. To mark a field as deprecated, use
-    `Record(field_name=Deprecated(type), ...)`. Multiple records can be
-    produced by returning an iterable of them. Registering generator functions
-    is currently not supported.
+    `Record(field_name=Deprecated(type), ...)`. Multiple records can be produced
+    by returning an iterable of them. Registering generator functions is
+    currently not supported.
 
     Example usage.
 
@@ -1459,8 +1458,9 @@ def transformation(func: typing.Callable[..., Record]):
 class FuncCtx:
     """Context of a function being executed.
 
-    Access to a FuncCtx is only valid during a single execution of a transformation.
-    You should not globally store a FuncCtx instance.
+    Access to a FuncCtx is only valid during a single execution of a function in
+    a query. You should not globally store a FuncCtx instance. The graph object
+    within the FuncCtx is not mutable.
     """
 
     __slots__ = "_graph"
@@ -1475,6 +1475,45 @@ class FuncCtx:
 
 
 def function(func: typing.Callable):
+    """
+    Register `func` as a user-defined function in the current module.
+
+    The decorator `function` is meant to be used to register module functions.
+    The registered `func` needs to be a callable which optionally takes
+    `FuncCtx` as its first argument. Other arguments of `func` will be bound to
+    values passed in the Cypher query. Only the funcion arguments need to be
+    annotated with types. The return type doesn't need to be specified, but it
+    has to be supported by `mgp.Any`. Registering generator functions is
+    currently not supported.
+
+    Example usage.
+
+    ```
+    import mgp
+    @mgp.function
+    def func_example(context: mgp.FuncCtx,
+        required_arg: str,
+        optional_arg: mgp.Nullable[str] = None
+        ):
+        return_args = [required_arg]
+        if optional_arg is not None:
+            return_args.append(optional_arg)
+        # Return any kind of result supported by mgp.Any
+        return return_args
+    ```
+
+    The example function above returns a list of provided arguments:
+      * `required_arg` is always present and its value is the first argument of
+        the function.
+      * `optional_arg` is present if the second argument of the function is not
+        `null`.
+    Any errors can be reported by raising an Exception.
+
+    The function can be invoked in Cypher using the following calls:
+      RETURN example.func_example("first argument", "second_argument");
+      RETURN example.func_example("first argument");
+    Naturally, you may pass in different arguments.
+    """
     raise_if_does_not_meet_requirements(func)
     register_func = _mgp.Module.add_function
     sig = inspect.signature(func)