Docs: Improve documentation for the `$prev_value` parameter and return result...

Docs: Improve documentation for the `$prev_value` parameter and return result of various metadata update functions.

Props stevenlinx.
Fixes #50502. See #49572.
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git-svn-id: https://core.svn.wordpress.org/trunk@47983 1a063a9b-81f0-0310-95a4-ce76da25c4cd