pgsql: Document limit on the number of out-of-line values per table

Document limit on the number of out-of-line values per table

Document the hard limit stemming from the size of an OID, and also
mention the perfomance impact that occurs before the hard limit
is reached.

Jakub Wartak and Robert Haas
Backpatch to all supported versions

Discussion: https://postgr.es/m/CAKZiRmwWhp2yxjqJLwbBjHdfbJBcUmmKMNAZyBjjtpgM9AMatQ%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4d93bbd4e0d414a33521f62d6249ac88486c866f

Modified Files
--------------
doc/src/sgml/limits.sgml | 11 +++++++++++
1 file changed, 11 insertions(+)