Clarify the documentation on how comparisons occur in an IN operator.

Fix the comparison operators when both sides of an IN operator are
expressions (ticket #2248).
Changes to main.mk for adding FTS2 into the standard build also
got mixed in with this check-in by mistake. (CVS 3656)

FossilOrigin-Name: da81725ca1cd894b3f2d734767e10cc0dc329566