Doc: improve explanation of GiST compress/decompress methods.

Doc: improve explanation of GiST compress/decompress methods.

The docs previously didn't explain that leaf and non-leaf keys
could be treated differently, even though many of our opclasses
do exactly that.  It also wasn't explained how that relates to
the STORAGE option, particularly since only one storage type
can be specified for both leaf and non-leaf keys.

While here, reorganize the text slightly, rather than sticking
additional detail into what's supposed to be a brief summary
paragraph.

Author: Paul A Jungwirth <pj@illuminatedcomputing.com>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA+renyWs5Np+FLSYfL+eu20S4U671A3fQGb-+7e22HLrD1NbYw@mail.gmail.com