pgsql: Add documentation chapter about checksums

Add documentation chapter about checksums

Data checksums did not have a longer discussion in the docs,
this adds a short section with an overview.

Extracted from the larger patch for on-line enabling of checksums, which
has many more authors and reviewers.

Author: Daniel Gustafsson
Reviewed-By: Magnus Hagander, Michael Banck (and others through the big patch)
Discussion: /messages/by-id/5ff49fa4.1c69fb81.658f3.04ac@mx.google.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cf621d9d84db1e6edaff8ffa26bad93fdce5f830

Modified Files
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doc/src/sgml/amcheck.sgml | 4 ++--
doc/src/sgml/ref/initdb.sgml | 1 +
doc/src/sgml/wal.sgml | 49 +++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 51 insertions(+), 3 deletions(-)