Bug 22098: (QA follow-up) Improving documentation

Bug 22098: (QA follow-up) Improving documentation

Improves the documentation of the script a bit, but makes one
important change:

-If no prefix is submitted, or prefix does not contain only
-numbers, it returns the inserted code (= keep a field unchanged)
+If no prefix is submitted, or the prefix does contain only
+numbers, it returns the inserted code (= keep the field unchanged).

To test:
- Add an authorised INVENTORY value with 2020 as prefix
- After linking the stocknumberAV.pl to the 952$i in the frameworks
- Edit an item
- Add 2020 in the stocknumber field
- Trigger plugin
- Verify nothing happens

If the prefix contains only numbers... nothing happens.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>