Use duplicate hook reference syntax for duplicates of 'link_category' and `pre_user_login`

[coffee2code]

link_category and pre_user_login appear to be the last remaining hooks that are defined in more than one place and still uniquely documented in each place. The convention for duplicated hooks is for one instance of the hook to be considered canonical with full documentation while all subsequent instances get documented with a special comment referencing that canonical documentation.

See attached patch for fix.

Related: the hook user_confirmed_action_email_content is also duplicated, but each instance's documentation differs in meaningful ways. See #44314 for discussion on the matter (particularly my pertinent comment).

[coffee2code]

Patch mentioned in ticket.

[desrosj]

Per today's bug scrub, documentation enhancements will be addressed in 5.1, coming next month.

[pento]

In 44612:

Docs: Remove duplicate docblocks for the pre_user_login and link_category filters.

Props coffee2code.
Fixes #45308.