pgsql: Code review for btree page split WAL reduction patch.

Log Message:
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Code review for btree page split WAL reduction patch.  Make it actually work
(original code *always* created a full-page image for the left page, thus
leaving the intended savings unrealized), avoid risk of not having enough room
on the page during xlog restore, squeeze out another couple bytes in the xlog
record, clean up neglected comments.

Modified Files:
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    pgsql/src/backend/access/nbtree:
        nbtinsert.c (r1.155 -> r1.156)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtinsert.c.diff?r1=1.155&r2=1.156)
        nbtxlog.c (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtxlog.c.diff?r1=1.42&r2=1.43)
    pgsql/src/include/access:
        nbtree.h (r1.112 -> r1.113)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/nbtree.h.diff?r1=1.112&r2=1.113)
    pgsql/src/include/catalog:
        catversion.h (r1.402 -> r1.403)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.402&r2=1.403)