Add the definition of PG temp to the glossary

it is not easy to figure out what a PG temp is just by reading the
code although it is easy to understand with an example.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #480 from dachary/master

Add the definition of PG temp to the glossary

Reviewed-by: Sage Weil <sage@inktank.com>