pgsql: Fix typos and grammar in documentation and code comments

Fix typos and grammar in documentation and code comments

Comment fixes are applied on HEAD, and documentation improvements are
applied on back-branches where needed.

Author: Justin Pryzby
Discussion: /messages/by-id/20210408164008.GJ6592@telsasoft.com
Backpatch-through: 9.6

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a2b35b3117eba20fc9d0410e0b737180d835595d

Modified Files
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doc/src/sgml/maintenance.sgml | 2 +-
doc/src/sgml/ref/createuser.sgml | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)