Document `-` stdin convention in CLI help text (#22817)
Summary
--
While responding to
https://github.com/astral-sh/ruff/discussions/13690#discussioncomment-15558697,
I realized that the `-` convention for reading from stdin wasn't
mentioned in
our CLI help anywhere. I thought it made sense to add a brief note to
the `FILES`
help section, but I'm happy to put it somewhere else or rephrase it if
someone
has a better suggestion!
Test Plan
--
Updated CLI tests
