pgsql: Correct several behavior descriptions in comments.

Correct several behavior descriptions in comments.

Reuse cautionary language from src/test/ssl/README in
src/test/kerberos/README.  SLRUs have had access to six-character
segments names since commit 73c986adde5d73a5e2555da9b5c8facedb146dcd,
and recovery stopped calling HeapTupleHeaderAdvanceLatestRemovedXid() in
commit 558a9165e081d1936573e5a7d576f5febd7fb55a.  The other corrections
are more self-evident.

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/676a9c3cc4b5f1d262c29de318868948513f0fa0

Modified Files
--------------
src/backend/access/heap/heapam.c                      |  2 --
src/backend/access/transam/README                     | 11 +++++------
src/backend/access/transam/varsup.c                   | 13 +++++++------
src/backend/commands/async.c                          | 11 ++++-------
src/backend/commands/vacuum.c                         | 10 +++++-----
src/backend/storage/buffer/bufmgr.c                   |  2 +-
src/bin/pg_waldump/pg_waldump.c                       | 11 +++--------
src/include/access/xlog_internal.h                    |  7 ++-----
src/test/kerberos/README                              | 10 ++++++----
src/test/perl/PostgresNode.pm                         |  6 ++----
src/test/recovery/t/010_logical_decoding_timelines.pl |  2 +-
src/test/ssl/t/SSLServer.pm                           |  1 -
12 files changed, 36 insertions(+), 50 deletions(-)