Rollup merge of rust-lang#31339 - rthomas:doc, r=alexcrichton

When trying to run a specific test, I found the contributing docs a bit confusing and through a bit of googling found out that TESTNAME takes the fully qual'd name of the test.

I'm unsure if this can also take the source file, but I was unable to get that to work.

Author: steveklabnik

CONTRIBUTING.md

@@ -131,8 +131,12 @@ Some common make targets are:
   & everything builds in the correct manner.
 - `make check-stage1-std NO_REBUILD=1` - test the standard library without
   rebuilding the entire compiler
-- `make check TESTNAME=<path-to-test-file>.rs` - Run a single test file
-- `make check-stage1-rpass TESTNAME=<path-to-test-file>.rs` - Run a single
+- `make check TESTNAME=<substring-of-test-name>` - Run a matching set of tests.
+  - `TESTNAME` should be a substring of the tests to match against e.g. it could 
+    be the fully qualified test name, or just a part of it. 
+    `TESTNAME=collections::hash::map::test_map::test_capacity_not_less_than_len`
+    or `TESTNAME=test_capacity_not_less_than_len`.
+- `make check-stage1-rpass TESTNAME=<substring-of-test-name>` - Run a single
   rpass test with the stage1 compiler (this will be quicker than running the
   command above as we only build the stage1 compiler, not the entire thing).
   You can also leave off the `-rpass` to run all stage1 test types.