Function: help-setup-xref

Invoked from commands using the "*Help*" buffer to install some xref info.

ITEM is a (FUNCTION . ARGS) pair appropriate for recreating the help
buffer after following a reference. INTERACTIVE-P is non-nil if the
calling command was invoked interactively. In this case the stack of
items for help buffer "back" buttons is cleared.

This should be called very early, before the output buffer is cleared,
because we want to record the "previous" position of point so we can
restore it properly when going back.