pgsql: doc: Clarify where autovacuum_max_parallel_workers can be set

doc: Clarify where autovacuum_max_parallel_workers can be set

The description of autovacuum_max_parallel_workers did not specify
where it can be set or how it relates to the per-table
autovacuum_parallel_workers storage parameter.

Clarify that it can be set only in postgresql.conf or on the server
command line, and that the limit can be reduced for individual tables.
Add a link to the autovacuum_parallel_workers storage parameter.

Backpatch to v19, where parallel autovacuum was introduced.

Author: Baji Shaik <baji(dot)pgdev(at)gmail(dot)com>
Reviewed-by: Sami Imseih <samimseih(dot)pg(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA+fm-RMmbUUPWJShxkhoB7SeiQ7XWw3YsVnHpMzp1Y2i_gtwig@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0a2e7a0b4c1ded0b9c062a26b6c76cad2bf14c3a

Modified Files
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doc/src/sgml/config.sgml | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)