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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1cdbf7f0d256b9147859643b20e812c0e9d10263

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(-)