howto-maintain: cover a whole development cycle
The "policy" part is more important than the "daily operation" part
in that it establishes why certain maintainer tasks exist and are
performed the way they are.
The text briefly touches the role each integration branches play in
the workflow, but does not give the whole picture of what happens in
a single development cycle using these branches. Extend the
description to describe a whole development cycle.
Signed-off-by: Junio C Hamano <gitster@pobox.com>