Docs: Add backtick escaping around a variable name in the DocBlock for `update_post_meta()`.

Also adds a note to the description explaining that it can be used in place of `add_post_meta()`.

See #42505.

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git-svn-id: https://core.svn.wordpress.org/trunk@44164 1a063a9b-81f0-0310-95a4-ce76da25c4cd