pgsql: Improve docs and error messages for parallel vacuum.

Improve docs and error messages for parallel vacuum.

The error messages, docs, and one of the options were using
'parallel degree' to indicate parallelism used by vacuum command. We
normally use 'parallel workers' at other places so change it for parallel
vacuum accordingly.

Author: Bharath Rupireddy
Reviewed-by: Dilip Kumar, Amit Kapila
Backpatch-through: 13
Discussion: https://postgr.es/m/CALj2ACWz=PYrrFXVsEKb9J1aiX4raA+UBe02hdRp_zqDkrWUiw@mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9012e5594c6ff803abb6d51e164797e3810845c1

Modified Files
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doc/src/sgml/ref/vacuumdb.sgml       | 6 +++---
src/backend/commands/vacuum.c        | 2 +-
src/bin/scripts/vacuumdb.c           | 4 ++--
src/test/regress/expected/vacuum.out | 2 +-
4 files changed, 7 insertions(+), 7 deletions(-)