4.1 Docs Audit: Ensure optional arguments in `get_the_post_navigation()`, `the_post_navigation()`, `get_the_posts_navigation()`, `the_posts_navigation()`, `get_the_pagination()`, and `the_pagination()` are properly documented with defaults.

See #30469.


git-svn-id: https://develop.svn.wordpress.org/trunk@30616 602fd350-edb4-49c9-b593-d223f7449a82