SWI-Prolog Search Documentation should support predicates in packs

Currently, SWI-Prolog Search Documentation does not support any predicates in packs. For example, reif.pl has if_/3, but you cannot search if_/3 in SWI-Prolog Search Documentation. This is a bit inconvenient.

IMO, Search Documentation should support any predicates in packs as long as those packs can be install by pack_install/1. AFAIK many languages do this thing (e.g. Racket website can search any procedures in packages as long as those packages can be installed by raco pkg install).