[libc++] Add documentation for the new comment-triggered benchmarking bot (#158167)

The new benchmarking bot is experimental and is very rough on the edges.

Author: ldionne

libcxx/docs/TestingLibcxx.rst

@@ -531,6 +531,16 @@ Finally, use ``compare-benchmarks`` to compare both:
 The ``compare-benchmarks`` script provides some useful options like creating a chart to easily visualize
 differences in a browser window. Use ``compare-benchmarks --help`` for details.
 
+Additionally, adding a comment of the following form to a libc++ PR will cause the specified benchmarks to be run
+on our pre-commit CI infrastructure and the results to be reported in the PR by our CI system:
+
+.. code-block::
+
+    /libcxx-bot benchmark <path/to/benchmark1.bench.cpp> <path/to/benchmark2.bench.cpp> ...
+
+Note that this is currently experimental and the results should not be relied upon too strongly, since
+we do not have dedicated hardware to run the benchmarks on.
+
 .. _`Google Benchmark`: https://github.com/google/benchmark
 
 .. _testing-hardening-assertions: