[FIX] payment_adyen: prevent creating a new session for returning users

Before this commit, users returning from Adyen to Odoo after payment
could see their session renewed, depending on their browser's
implementation of the `SameSite` cookie attribute. This prevented Odoo
from retrieving the transaction from the users' session.

This commit flags the return route of Adyen with `save_session=False`,
hence allowing all users to immediately post-process their transactions
when they return to Odoo.

While we're at it, the docstrings of the return routes of PayUmoney and
SIPS' have been updated for better clarity.

closes odoo/odoo#74763

Signed-off-by: Antoine Vandevenne (anv) <AntoineVDV@users.noreply.github.com>