pgsql: Fix the usage of parallel and full options of vacuum command.

Fix the usage of parallel and full options of vacuum command.

Earlier we were inconsistent in allowing the usage of parallel and
full options. Change it such that we disallow them only when they are
combined in a way that we don't support.

In passing, improve the comments in some of the existing tests of parallel
vacuum.

Reported-by: Tushar Ahuja
Author: Justin Pryzby, Amit Kapila
Reviewed-by: Sawada Masahiko, Michael Paquier, Mahendra Singh Thalor and
Amit Kapila
Discussion: https://postgr.es/m/58c8d171-e665-6fa3-a9d3-d9423b694dae%40enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/24d2d38b1eb86c0b410ad0f07f66566a83c6f05c

Modified Files
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doc/src/sgml/ref/vacuum.sgml | 32 ++++++++++++++++----------------
src/backend/commands/vacuum.c | 6 ++----
src/test/regress/expected/vacuum.out | 6 ++++--
src/test/regress/sql/vacuum.sql | 5 ++++-
4 files changed, 26 insertions(+), 23 deletions(-)