Brief outline of the bug

This is the same as latex3/latex3#1345 but for ltfilehook-code and ltshipout-code, namely undefined page references in the sentences "These functions are documented on page ??". I think fixing it should just require adding the documented-as, no-user-doc, or deprecated options in the right places. Here's the list of where the undefined references occur.

ltfilehook-code

• \InputIfFileExists, \@input@file@exists@with@hooks, and \unqu@tefilef@und
• \set@curr@file and others
• \@filehook@set@CurrentFile, \@kernel@make@file@csname, and \@set@curr@file@aux
• \@@_set_curr_file:nNN and \@@_set_curr_file_assign:nnnNN
• \BeforeClearDocument

ltshipout-code

• \@kernel@after@shipout@lastpage, \@kernel@before@shipout@background, and \@kernel@after@shipout@background
• \@abspage@last
• \enddocument, \@kernel@after@enddocument, and \@kernel@after@enddocument@afterlastpage
• \@expl@@@shipout@add@firstpage@material@@Nn and others
• \ShipoutBoxHeight, \ShipoutBoxWidth, and \ShipoutBoxDepth