Add warning about saving container fields

As https://www.tryton.org/~irclog/2022-12-06.log.html#t09:12-3 shows it is not clear how such fields can be saved.

assigned to @ced

added trytond ~844 + 1 deleted label

added 1 deleted label and removed 1 deleted label

New review422021008 at https://codereview.tryton.org/422021008/#ps416011003

mentioned in commit 77383f5f178e

mentioned in merge request !33 (merged)

mentioned in commit 6a385ecfb339

mentioned in commit ad1cddf183c9

mentioned in commit 4201a0634ff8

mentioned in commit 83f4a4a05c34

closed with merge request !33 (merged)