Docs: Add a `@since` note to `get_the_archive_title()` about the title part...

Docs: Add a `@since` note to `get_the_archive_title()` about the title part being wrapped in a `<span>` element.

Follow-up to [48294].

Props justinahinon, audrasjb.
Merges [48814] to the 5.5 branch.
Fixes #51050.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48577 1a063a9b-81f0-0310-95a4-ce76da25c4cd