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