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>
- fix discussion of last epoch started
- define terms for current and past intervals
- describe role of pg info
- remove mention of the backlog
- fix discussion of up_thru
- etc.
Signed-off-by: Sage Weil <sage@newdream.net>