[FIX] *mail_thread: don't modify modified help

The 'help' of window action is often fiddled with, adding thing before,
after or arround it.

For example in CRM leads, we add at the beginning "Click to add a new
opportunity" with an arrow towards the button, and after if there is a
mail alias: "All email incoming to * will automatically create new...".

But for crm.lead, hr.expense, sale.order this would not take into
account that the fiddled "help" can be edited, so if we edit 2 times
help in studio or backend action editing, we would get:

  Click to add a new opportunity
  Click to add a new opportunity
  Click to add a new opportunity
  [Original help content]
  All email incoming to * will automatically create new...
  All email incoming to * will automatically create new...
  All email incoming to * will automatically create new...

And see several "arrows" towards the button (in enterprise the
additional ones are on same color background).

With this commit we do what is done in "mail.thread" by default which is
not fiddling with the `help` if has been fiddled before (if it contains
"oe_view_nocontent_create" class).

note: for 10.0 up to not including 11.0 which is fixed by #26911

opw-1877663
closes #26912