bug#34150: 26.1; Document filtering with `isearch-filter-predicate' in Elisp manual

[Drew Adams]

> > But it and its consequences are not necessarily
> > obvious - especially to a user searching, as opposed
> > to a programmer writing a filter predicate.
> 
> This is not a user-level facility, so the user perspective is not
> relevant.

The "user perspective" is always relevant for Emacs.
We are all users.

Searching is a user-level facility.  If a search
imposes/provides filtering, that certainly affects
user-visible behavior.  Users should understand
that behavior.

Anyone imposing such filtering on users should
consider its effects on users, including the
considerations raised by this bug report.  That
starts with making the behavior and consequences
clear to filter implementors.