pgsql: Fix comments about WAL rule "write xlog before data" versus pg_m

Fix comments about WAL rule "write xlog before data" versus pg_multixact.

Recovery does not achieve its goal of zeroing all pg_multixact entries
whose accompanying WAL records never reached disk. Remove that claim
and justify its expendability. Detail the need for TrimMultiXact(),
which has little in common with the TrimCLOG() rationale. Merge two
tightly-related comments. Stop presenting pg_multixact as specific to
heap_lock_tuple(); PostgreSQL 9.3 extended its use to heap_update().

Noticed while investigating a report from Andres Freund.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/3cd1ba147e5619199914e5b71e0edbd188a763d2

Modified Files
--------------
src/backend/access/transam/multixact.c | 46 +++++++++++++++-----------------
1 file changed, 21 insertions(+), 25 deletions(-)