PR Summary

• remove '(default)' from cap style demo as this is only true for Line2D
  and the default rcParameters
• document default cap styles for Line2D and Patch in their cap style
  setters
• document default cap style for GraphicsContextBase in the same way as
  it's already done for joinstyle

Closes#21979. If this goes in I can do the same for join styles, see#18597 (just documenting the current state).

PR Checklist

Tests and Styling

• [N/A] Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (installflake8-docstrings and runflake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [N/A] New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).