Add docs for ThreadRange. (#1318)

Fixes #1297

docs/user_guide.md

@@ -773,6 +773,16 @@ static void BM_MultiThreaded(benchmark::State& state) {
 BENCHMARK(BM_MultiThreaded)->Threads(2);
 ```
 
+To run the benchmark across a range of thread counts, instead of `Threads`, use
+`ThreadRange`. This takes two parameters (`min_threads` and `max_threads`) and
+runs the benchmark once for values in the inclusive range. For example:
+
+```c++
+BENCHMARK(BM_MultiThreaded)->ThreadRange(1, 8);
+```
+
+will run `BM_MultiThreaded` with thread counts 1, 2, 4, and 8.
+
 If the benchmarked code itself uses threads and you want to compare it to
 single-threaded code, you may want to use real-time ("wallclock") measurements
 for latency comparisons: