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