gh-142927: Clarify pstats file output in docs and CLI (#143388)

When running the `profiling.sampling` module in pstats mode, the output
can be emitted in two different ways: text to stdout or a binary file
when the `--output` argument is set.

The current documentation and help text is confusing as it does not
distinguish between these two output formats so it may be surprising to
the user to get different formats depending whether `--output` is set or not.