doc: clarify migration steps

[thiswillbeyourgithub]

Signed-off-by: thiswillbeyourgithub [email protected]

Hi,

I did the migration to AIO but wanted to clarify a few points:

1. put more emphasis on when it is expected to let the containers off
2. remove the duplicate line asking to upgrade the apps, we are already supposed to do it in one of the earlier steps. Moreover the term "webinterface" is unclear: is that the AIO or nextcloud interface? Also note that newcomers just discovered the AIO UI for the first time so it's not yet clear if we can or can't add apps from it.

[szaimen]

Hi, thanks for the PR! I have one comment below

[szaimen]

LGTM, thanks!