Docstring cleanup: "default None", doi

[echedey-ls]

• Closes "default None" in docstring parameter descriptions considered harmful #1574 . All changes at the bottom.
• I am familiar with the contributing guidelines
• [ ] Tests added
• [ ] Updates entries in docs/sphinx/source/reference for API changes.
• [ ] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• Current code includes numpydoc compliant docstrings, examples, and comments everywhere.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Cherry-picked commits from #1790 and repeated the process with the regexes, with little changes to them.

Changes in this PR

1. Changes excessive use of parentheses in defaults
   return_components: bool (optional, default=False) -> return_components : bool, default False
2. default None -> optional ("default None" in docstring parameter descriptions considered harmful #1574)
3. If None -> If not specified
4. Inexistent DOI roles are now roles, e. g.: DOI: 10... -> :doi:`10...`