show_defaults now accepts string value as suggested by the docs

[b3n4kh]

Fixes #1732 by removing the mandatory "default" flag for using show_defaults with a string value.

• Fixes Custom default label ignored if default is None #1732

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [docs already up2date] Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

If Option.show_default or Context.show_default is True, this change causes it to show None instead of hiding the value. Needs some extra checks to only show when it's a string.