Document that get_metadata() returns false for bad parameters

[salcode]

Currently the inline documentation for the return value for get_metadata() reads

@return mixed Single metadata value, or array of values

I've noticed that if you pass in a post id of zero, you get a value of false. While the value of false falls under the category of "mixed", I would like to see it mentioned explicitly.

If we make this change, we should do the same for any function that uses get_metadata(), e.g. get_post_meta()

[salcode]

Here is the [GitHub branch](​https://github.com/salcode/wordpress-develop/tree/sf/get-metadata-doc-40519) where I've made this change.

[kirasong]

Hi @salcode!

Apologies for the very slow reply on this, and thanks much for your work and patch.

We chatted about this ticket in today's ​ticket triage.

It looks like these changes would still help make the documentation clearer, but unfortunately the patch doesn't apply anymore.

If you, or anyone else, has a chance to refresh the patch, it would be much appreciated.

[johnbillion]

This was fixed in [48192] and [48411].

[SergeyBiryukov]

In 48658:

Docs: Document that various metadata functions return false for an invalid ID.

Props salcode.
See #40519.