Correct several behavior descriptions in comments.

Noah Misch <noah@leadboat.com>

Commit: 676a9c3cc4b5f1d262c29de318868948513f0fa0
Author: Noah Misch <noah@leadboat.com>
Date: 2020-08-16T03:21:52Z
Releases: 14.0
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.

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