Clarify return results for a non-existing ID in metadata functions

[SergeyBiryukov]

Background: #40519

[48658] documented that various metadata functions return false for an invalid ID. However, that does not clarify what an invalid ID is: a non-numeric, zero, or negative value.

For example, if a valid but non-existing post ID is passed, get_post_meta() returns an empty string, not false, but it's not mentioned in the documentation.

This applies to:

• get_comment_meta()
• get_site_meta()
• get_post_meta()
• get_term_meta()
• get_user_meta()

And potentially these functions too:

• get_metadata()
• get_metadata_raw()
• get_metadata_default()

Props to @icopydoc for the ​report on support forums.

[david.kryzaniak]

Updates to docs

[audrasjb]

Thanks @davidkryzaniak. In 51797.2.diff I completed the docs with more details and I also updated get_metadata() and get_metadata_raw(). I didn't change get_metadata_default() because I'm not sure this comment for the returned value is relevant for this function.

[SergeyBiryukov]

Thanks for the patch!

Looks like this needs some minor adjustments to replace "post ID" with "term ID", "user ID", etc. where appropriate. Moving to the next release for now for a closer review.

[SergeyBiryukov]

Thanks for the patch! This mostly looks good.

51797.4.diff​ makes some further adjustments for consistency and accuracy.

[davidbaumwald]

In 50641:

Docs: Clarify return results for a non-existing ID in metadata functions.

[48658] documented that various metadata functions return false for an invalid ID. However, that does not clarify what an invalid ID is: a non-numeric, zero, or negative value. This change adds the clarification in all relevant metadata function docblocks.

Props icopydoc, SergeyBiryukov, davidkryzaniak, audrasjb.
Fixes #51797.