pgsql: Doc: Fix misleading statement about VACUUM memory limits

Doc: Fix misleading statement about VACUUM memory limits

In ec34040af I added a mention that there was no point in setting
maintenance_work_limit to anything higher than 1GB for vacuum, but that
was incorrect as ginInsertCleanup() also looks at what
maintenance_work_mem is set to during VACUUM and that's not limited to
1GB.

Here I attempt to make it more clear that the limitation is only around
the number of dead tuple identifiers that we can collect during VACUUM.

I've also added a note to autovacuum_work_mem to mention this limitation.
I didn't do that in ec34040af as I'd had some wrong-headed ideas about
just limiting the maximum value for that GUC to 1GB.

Author: David Rowley
Discussion: https://postgr.es/m/CAApHDvpGwOAvunp-E-bN_rbAs3hmxMoasm5pzkYDbf36h73s7w@mail.gmail.com
Backpatch-through: 9.6, same as ec34040af

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/44ca43e36c54d30ed789d7e0544efeb5a5eb6c3b

Modified Files
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doc/src/sgml/config.sgml | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)