Code cleanup, improve docs

mirai-core-api/src/commonMain/kotlin/message/data/Message.kt

@@ -54,9 +54,6 @@ import kotlin.contracts.contract
  *  ```
  * 但注意: 不能 `String + Message`. 只能 `Message + String`
  *
- * #### 实现规范
- * 除 [MessageChain] 外, 所有 [Message] 的实现类都有伴生对象实现 [Key] 接口.
- *
  * #### 发送消息
  * - 通过 [Contact] 中的成员函数: [Contact.sendMessage]
  * - 通过 [Message] 的扩展函数: [Message.sendTo]
@@ -130,6 +127,7 @@ public interface Message { // must be interface. Don't consider any changes.
      * - ...
      *
      * @see toString 得到包含 mirai 消息元素代码的, 易读的字符串
+     * @see Message.content Kotlin 扩展
      */
     public fun contentToString(): String
 

mirai-core-api/src/commonMain/kotlin/message/data/impl.kt

@@ -118,7 +118,6 @@ internal fun Sequence<SingleMessage>.constrainSingleMessages(): List<SingleMessa
 /**
  * - [Sequence.toMutableList]
  * - Replace in-place with marker null
- * - [Iterable.filterNotNull]
  */
 @MiraiExperimentalApi
 @JvmSynthetic
@@ -147,15 +146,6 @@ internal fun constrainSingleMessagesImpl(sequence: Sequence<SingleMessage>): Lis
 internal fun Iterable<SingleMessage>.constrainSingleMessages(): List<SingleMessage> =
     constrainSingleMessagesImpl(this.asSequence())
 
-@JvmSynthetic
-internal inline fun <T> List<T>.indexOfFirst(offset: Int, predicate: (T) -> Boolean): Int {
-    for (index in offset..this.lastIndex) {
-        if (predicate(this[index]))
-            return index
-    }
-    return -1
-}
-
 
 @JvmSynthetic
 @Suppress("UNCHECKED_CAST", "DEPRECATION_ERROR", "DEPRECATION")
@@ -184,12 +174,6 @@ internal data class MessageChainImpl constructor(
     override fun equals(other: Any?): Boolean = other is MessageChainImpl && other.delegate == this.delegate
 }
 
-@Suppress("FunctionName")
-internal fun CombinedMessage(
-    left: Message,
-    tail: Message
-): MessageChain = MessageChainImplBySequence(left.flatten() + tail.flatten())
-
 @Suppress("FunctionName") // source compatibility with 1.x
 internal fun MessageChainImplBySequence(
     delegate: Sequence<SingleMessage> // 可以有重复 ConstrainSingle
@@ -266,7 +250,6 @@ internal fun calculateImageMd5ByImageId(imageId: String): ByteArray {
     }
 }
 
-@PublishedApi
 internal val ILLEGAL_IMAGE_ID_EXCEPTION_MESSAGE: String =
     "ImageId must match Regex `${FRIEND_IMAGE_ID_REGEX_1.pattern}`, " +
             "`${FRIEND_IMAGE_ID_REGEX_2.pattern}` or " +