pg_dump docs should mention TMPDIR

A suggested addition to the pg_dump docs:

This would be an extension to the documentation for the "tar" format:

The tar format needs some space to create temporary files. By default
"/tmp" is used. On systems with small "/tmp" partitions, set the
"TMPDIR" environment variable to a location with more space, such as
"/usr/tmp".

Also, I would like it if the pg_dump docs more assertively recommended
one of the "tar" or "custom" formats. They seem very similiar.

It woud also be nice to document that the full names "custom" and "tar" are
supported. Longer names can be nice for clarity.

( Unfortunately, wrong formats like "txx" also work instead of throwing
an error. )

    Mark