pgsql: Update some incorrect comments about xlog records.

Update some incorrect comments about xlog records.

The comments claim that certain pieces of data are part of the main
WAL record data when in reality they are part of the data for
block 0. Repair.

Bertrand Drouvot, reviewed by Amit Kapila. Originally reported by me.

Discussion: http://postgr.es/m/80db7836-4415-d54a-64c3-66b88b1430e7@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ebd551f586a801dee426e49ba72fb213e8013890

Modified Files
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src/include/access/gistxlog.h    |  4 +---
src/include/access/heapam_xlog.h |  5 +++--
src/include/access/nbtxlog.h     | 10 +++++++---
3 files changed, 11 insertions(+), 8 deletions(-)