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
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2f0491dd1a34ea9390910afe532b6c2ca8db3c11

Modified Files
--------------
src/backend/access/gist/gist.c | 8 +-
src/backend/access/gist/gistsplit.c | 439 +++++++++++++++++++++++------------
src/include/access/gist.h | 29 ++-
src/include/access/gist_private.h | 13 +-
4 files changed, 319 insertions(+), 170 deletions(-)