Incorrect documented return type for bookmark functions add_link(), edit_link(), and wp_update_link()

[marian1]

The functions add_link(), edit_link(), and wp_update_link() are documented as returning int|WP_Error.

In practice, they never return a WP_Error instance. All three ultimately call wp_insert_link(), whose second parameter ($wp_error) controls whether failures are returned as WP_Error or as 0. The parameter defaults to false, and none of these wrapper functions passes true (nor do they expose a parameter to allow it).

As a result, these functions return an integer in all cases: 0 on failure, and the link ID on success. Their @return tags should therefore be updated to int.

Trac ticket: https://core.trac.wordpress.org/ticket/64764

## Use of AI Tools

[SergeyBiryukov]

In 62024:

Docs: Correct return type for add_link(), edit_link(), and wp_update_link().

Follow-up to [8758], [15564].

Props marian1.
Fixes #64764.

Added props for @apermo and myself in Core Props.