[IMP] doc: add list of possible values for license key

The list comes from the selection values on the field license of
ir.module.module

closes odoo/odoo#39354

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

doc/reference/module.rst

@@ -51,7 +51,20 @@ Available manifest fields are:
 ``website`` (``str``)
     website URL for the module author
 ``license`` (``str``, defaults: ``LGPL-3``)
-    distribution license for the module
+    distribution license for the module.
+    Possible values:
+
+    * `GPL-2`
+    * `GPL-2 or any later version`
+    * `GPL-3`
+    * `GPL-3 or any later version`
+    * `AGPL-3`
+    * `LGPL-3`
+    * `Other OSI approved licence`
+    * `OEEL-1` (Odoo Enterprise Edition License v1.0)
+    * `OPL-1` (Odoo Proprietary License v1.0)
+    * `Other proprietary`
+
 ``category`` (``str``, default: ``Uncategorized``)
     classification category within Odoo, rough business domain for the module.