Incorrect @return tag in doc comment for get_post_field()

[marian1]

get_post_field() either returns an empty string or the sanitised post field value via sanitize_post_field().

Although the documented return type is string, the actual return value and type depend on the requested post field. For example, get_post_field('post_parent', $post_id) returns an integer value.

To align the documentation with the actual behaviour, I suggest updating the @return tag to either mixed (as documented for sanitize_post_field()) or specifying the union of all possible return types.

Update get_post_field() docblock to reflect actual return types (int|string|int[]) instead of always string.

Trac ticket: https://core.trac.wordpress.org/ticket/63802

[SergeyBiryukov]

In 60621:

Docs: Correct @return type for get_post_field().

Follow-up to [6379], [21559], [24490].

Props marian1, rollybueno.
Fixes #63802.