pgsql: Clean up and document the API for XLogOpenRelation and

Log Message:
-----------
Clean up and document the API for XLogOpenRelation and XLogReadBuffer.
This commit doesn't make much functional change, but it does eliminate some
duplicated code --- for instance, PageIsNew tests are now done inside
XLogReadBuffer rather than by each caller.
The GIST xlog code still needs a lot of love, but I'll worry about that
separately.

Modified Files:
--------------
    pgsql/src/backend/access/gist:
        gistxlog.c (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistxlog.c.diff?r1=1.11&r2=1.12)
    pgsql/src/backend/access/heap:
        heapam.c (r1.209 -> r1.210)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.209&r2=1.210)
    pgsql/src/backend/access/nbtree:
        nbtxlog.c (r1.28 -> r1.29)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtxlog.c.diff?r1=1.28&r2=1.29)
    pgsql/src/backend/access/transam:
        README (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/README.diff?r1=1.3&r2=1.4)
        xact.c (r1.218 -> r1.219)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.218&r2=1.219)
        xlog.c (r1.229 -> r1.230)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.229&r2=1.230)
        xlogutils.c (r1.41 -> r1.42)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlogutils.c.diff?r1=1.41&r2=1.42)
    pgsql/src/backend/commands:
        dbcommands.c (r1.178 -> r1.179)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.178&r2=1.179)
        sequence.c (r1.130 -> r1.131)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/sequence.c.diff?r1=1.130&r2=1.131)
        tablespace.c (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablespace.c.diff?r1=1.33&r2=1.34)
    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.204 -> r1.205)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.204&r2=1.205)
    pgsql/src/backend/utils/init:
        postinit.c (r1.161 -> r1.162)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c.diff?r1=1.161&r2=1.162)
    pgsql/src/include/access:
        xlogutils.h (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlogutils.h.diff?r1=1.19&r2=1.20)
    pgsql/src/include/storage:
        bufmgr.h (r1.98 -> r1.99)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufmgr.h.diff?r1=1.98&r2=1.99)