Remove obsolete link to benchmarking tests

The benchmarking feature is now documented, so there is no need
to direct our users towards our test file to understand how it
works.

Closes #2471

docs/test-cases-and-sections.md

@@ -72,7 +72,8 @@ All tag names beginning with non-alphanumeric characters are reserved by Catch.
 
 * `[@<alias>]` - tag aliases all begin with `@` (see below).
 
-* `[!benchmark]` - this test case is actually a benchmark. This is an experimental feature, and currently has no documentation. If you want to try it out, look at `projects/SelfTest/Benchmark.tests.cpp` for details.
+* `[!benchmark]` - this test case is actually a benchmark. Currently this only serves to hide the test case by default, to avoid the execution time costs.
+
 
 ## Tag aliases