Docs: Update documentation for `get_custom_logo()` and `the_custom_logo()` to...

Docs: Update documentation for `get_custom_logo()` and `the_custom_logo()` to mention the `unlink-homepage-logo` theme feature.

Follow-up to [48039], [48283], [48749], [48757], [48758].

Props sabernhardt.
Merges [48832] to the 5.5 branch.
Fixes #51075.
Built from https://develop.svn.wordpress.org/branches/5.5@48833


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48595 1a063a9b-81f0-0310-95a4-ce76da25c4cd