Update docblock for is_tax to indicate it's for custom taxonomies only

[theMikeD]

We should add the info that is_tax is for custom taxonomies only now that devhub pulls its primary content direct from source.

[theMikeD]

Patch for 36331

[SergeyBiryukov]

36331.2.diff​ also updates WP_Query::is_tax() docs.

[theMikeD]

Missed that one. Thanks @SergeyBiryukov !

[SergeyBiryukov]

In 37235:

Docs: Clarify that is_tax() and WP_Query::is_tax() return false for built-in taxonomies (category and tag archives).

Props theMikeD.
Fixes #36331.

[DrewAPicture]

The false return condition info should be added directly to the return description for this function and method instead of in the DocBlock description.

[SergeyBiryukov]

In 37281:

Docs: Move the clarification of is_tax() and WP_Query::is_tax() return value added in [37235] to @return description.

Props DrewAPicture.
Fixes #36331.