Doc: rework introductory documentation about covering indexes.

Documenting INCLUDE in the section about unique indexes is confusing,
as complained of by Emilio Platzer.  Furthermore, it entirely failed
to explain why you might want to use the feature.  The section about
index-only scans is really the right place; it already talked about
making such things the hard way.  Rewrite that text to describe INCLUDE
as the normal way to make a covering index.

Also, move that section up a couple of places, as it now seems more
important than some of the stuff we had before it.  It still has to
be after expression and partial indexes, since otherwise some of it
would involve forward references.

Discussion: https://postgr.es/m/154031939560.30897.14677735588262722042@wrigleys.postgresql.org