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 73c986adde,
and recovery stopped calling HeapTupleHeaderAdvanceLatestRemovedXid() in
commit 558a9165e0.  The other corrections
are more self-evident.

src/backend/storage/buffer/bufmgr.c

@@ -3578,7 +3578,7 @@ IncrBufferRefCount(Buffer buffer)
  * This is essentially the same as MarkBufferDirty, except:
  *
  * 1. The caller does not write WAL; so if checksums are enabled, we may need
- *	  to write an XLOG_FPI WAL record to protect against torn pages.
+ *	  to write an XLOG_FPI_FOR_HINT WAL record to protect against torn pages.
  * 2. The caller might have only share-lock instead of exclusive-lock on the
  *	  buffer's content lock.
  * 3. This function does not guarantee that the buffer is always marked dirty