pgsql: doc: Improve some grammar and sentences

doc: Improve some grammar and sentences

90fbf7c has taken care of that for HEAD.  This includes the portion of
the fixes that applies to the documentation, where needed depending on
the branch.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20201227202604.GC26311@telsasoft.com
Backpatch-through: 9.5

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5253906fac5a2f3669f7867bcb5507f6f0ea891c

Modified Files
--------------
doc/src/sgml/catalogs.sgml            | 2 +-
doc/src/sgml/pgstatstatements.sgml    | 2 +-
doc/src/sgml/ref/explain.sgml         | 6 +++---
doc/src/sgml/ref/pg_dump.sgml         | 2 +-
doc/src/sgml/ref/pg_verifybackup.sgml | 2 +-
doc/src/sgml/sources.sgml             | 2 +-
doc/src/sgml/wal.sgml                 | 2 +-
7 files changed, 9 insertions(+), 9 deletions(-)