doc: process: Correct and expand slightly on the Merge Window concept

For quite a long time we've been using a 3 week, rather than 2 week,
merge window as it was only 2 weeks during the timeframe where we did 2
month rather than 3 month releases.  This corrects the places that still
had 2 weeks and tries to make things a bit clearer overall.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>