pgsql: Doc: mention limitation of the number of resultset columns

Doc: mention limitation of the number of resultset columns

The PostgreSQL limitations section of the documents mentioned the limit
on the number of columns that can exist in a table.  Users might be
surprised to find that there's also a limit on the number of columns that
can exist in a targetlist.  Users may experience restrictions which
surprise them if they happened to select a large number of columns from
several tables with many columns.  Here we document that there is a
limitation on this and mention what that limit actually is.

Wording proposal by Alvaro Herrera

Reported-by: Vladimir Sitnikov
Author: Dave Crammer
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAB=Je-E18aTYpNqje4mT0iEADpeGLSzwUvo3H9kRRuDdsNo4aQ@mail.gmail.com
Backpatch-through: 12, where the limitations section was added

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e6bd7aafc82d5f7ae1cb4bf15ce47d2ad12e5dd3

Modified Files
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doc/src/sgml/limits.sgml | 6 ++++++
1 file changed, 6 insertions(+)