Improve description of check command.

[mauvilsa]

Creating this pull request as proposed in #1152 (comment) by @woodruffw. It modifies the description of the check command such that:

• From command line twine check --help also shows the complete description.
• The description is improved so that it is more clear about the current purpose of check.

[sigmavirus24]

The docstring doesn't need updating unless I'm mistaken

[mauvilsa]

Some people might look at twine check --help, but others might look at the docs, i.e. twine.commands.check.check. From what I understand only the docstrings are shown there and not the new description, so it does make sense to also change it.

[sigmavirus24]

That is specifically listed internally as part of contributor guidelines. No end user should be relying on the docstring for understanding how this command works.

[mauvilsa]

I looked at the contributor guidelines and didn't see anything about docstrings, so not really sure what you mean. I have no problem in removing that change. But note, not all end users are familiar with CLIs or them normally having a --help option. Just because somewhere it says that end users should not rely on what is in the official docs https://twine.readthedocs.io/, doesn't mean they won't. On the other hand, the comment about the long_description was already in the docstring. Do you mean to say that it shouldn't be there?