pgsql: Document and clean up gistsplit.c.

Document and clean up gistsplit.c.

Improve comments, rename some variables and functions, slightly simplify
a couple of APIs, in an attempt to make this code readable by people other
than its original author.

Even though this is essentially just cosmetic, back-patch to all active
branches, because otherwise it's going to make back-patching future fixes
in this file very painful.

Branch
------
REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/adc3fdd0d0e19d384fe71d7bd4b1df94d0fa30e7

Modified Files
--------------
src/backend/access/gist/gist.c      |    8 +-
src/backend/access/gist/gistsplit.c |  442 +++++++++++++++++++++++------------
src/include/access/gist.h           |   29 ++-
src/include/access/gist_private.h   |   13 +-
4 files changed, 320 insertions(+), 172 deletions(-)