pgsql: doc: Add appendix detailing some limits of PostgreSQL

doc: Add appendix detailing some limits of PostgreSQL

This used to be on the web site but was removed.  The documentation is
a better place for it anyway.

Author: David Rowley <david.rowley@2ndquadrant.com>
Reviewed-by: John Naylor <jcnaylor@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CAKJS1f_dKdejdKB94nKZC9S5NzB-UZRcAKkE84e=JEEecDuotg@mail.gmail.com/

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/68120427f4902c136066d10ac9da93355877e156

Modified Files
--------------
doc/src/sgml/filelist.sgml |   1 +
doc/src/sgml/limits.sgml   | 120 +++++++++++++++++++++++++++++++++++++++++++++
doc/src/sgml/postgres.sgml |   1 +
3 files changed, 122 insertions(+)