pgsql: Further dtrace adjustments for the backend-IDs-in-relpath patch.

Log Message:
-----------
Further dtrace adjustments for the backend-IDs-in-relpath patch.

Update the documentation, and back out a few ill-considered changes
whose folly I failed to realize for failure to read the documentation.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        monitoring.sgml (r1.82 -> r1.83)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml?r1=1.82&r2=1.83)
    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.258 -> r1.259)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c?r1=1.258&r2=1.259)
    pgsql/src/backend/utils:
        probes.d (r1.14 -> r1.15)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/probes.d?r1=1.14&r2=1.15)