Link to an issue with more discussion.

It's only used to set markdown_text, whose value can safely be used
outside of the with block.

Behavior and thus implementation is nearly identical to that of the
--description option.

Unlike --description, it makes sense to keep --warning short. Users may
find it easier to incorporate --warning into workflows if it allows a
single string instead of requiring a Markdown file.

Added quotes to the test since those seem the most likely to stop
working upon future code changes.