Auto merge of #5341 - dwijnand:test-help, r=alexcrichton

tweak test/bench's help around filtering & libtest

Attempt to bring more clarity about test/bench's filtering arg by
detailing the distinction and integration between cargo's test/bench and
libtest.

Also make sure to mention how to see libtest's help/usage message.

Fixes #5337

Feedback welcome.  Also, how do I auto-generate the man pages in
src/etc/man/?  I'm wasn't sure about the escaping to attempt manually
editing.