BUG #12086: docs: prewarm: When does prewarm happen?

The following bug has been logged on the website:

Bug reference:      12086
Logged by:          Thomas Güttler
Email address:      guettliml@thomas-guettler.de
PostgreSQL version: 9.4beta2
Operating system:   none
Description:

The docs of prewarm could be improved:

http://www.postgresql.org/docs/9.4/static/pgprewarm.html

I am missing: When does the prewarm happen? I guess it happens at server
start. But that's just a guess. Or does postgres do prewarm every N minutes?
Please improve the docs.

Second thing I am missing: How to use this method? Please provide an
example.

Thank you.