org: allow relative time in `org-add-planning-info'.

org: allow relative time in `org-add-planning-info'.

(org-deadline, org-schedule): Improve docstrings.
(org-add-planning-info): Allow relative time like "-2d" or
"++2w" for the TIME argument.

The user can now call `org-schedule' and `org-deadline'
and use "+1d" to say that the task must be scheduled one
day later than today, or "++1d" to say that the task must
be scheduled one day later than the current SCHEDULED date,
if any.

This also works in agenda: you can select several entries
and reschedule them to +2d.

Thanks to Memnon Anon for insisting an finding a solution
for this!