pgsql: Correct the descriptions of recovery-related wait events in docs

Correct the descriptions of recovery-related wait events in docs.

This commit corrects the descriptions of RecoveryWalAll and RecoveryWalStream
wait events in the documentation.

Back-patch to v10 where those wait events were added.

Author: Fujii Masao
Reviewed-by: Kyotaro Horiguchi, Atsushi Torikoshi
Discussion: https://postgr.es/m/124997ee-096a-5d09-d8da-2c7a57d0816e@oss.nttdata.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f4efb46d2230ef19165fec78b5c41a78cf6e1213

Modified Files
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doc/src/sgml/monitoring.sgml | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)