In commit f5d8b5e (Support durations in recurrence ending dates,
2016-02-25), we added support for the `+xxwxxd` syntax when specifying
recurrence end dates. However, this only worked when *editing* the
recurrence, not when converting a regular item to a recurrent one. Fix
this and support the new syntax in both cases.
Also, mention the new syntax in the prompt.
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>