pgsql: Doc: Review for "Optionally prefetch referenced data in recovery

Doc: Review for "Optionally prefetch referenced data in recovery."

Typos, corrections and language improvements in the docs, and a few in
code comments too.

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20210409033703.GP6592%40telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/dc88460c24ed71ba7464ef4749e5f25da1bf6652

Modified Files
--------------
doc/src/sgml/config.sgml                  |  2 +-
doc/src/sgml/wal.sgml                     |  4 +---
src/backend/access/transam/xlogprefetch.c | 12 +++++++-----
src/backend/utils/misc/guc.c              |  2 +-
4 files changed, 10 insertions(+), 10 deletions(-)