[OpenSIPS-Devel] [OpenSIPS/opensips] d2b911: avpops: document the returned value in case of NULL

[Răzvan Crainea]

  Branch: refs/heads/3.3
  Home:   https://github.com/OpenSIPS/opensips
  Commit: d2b911ff53cc226ec657e3f73f1ff3727c1542fe
      https://github.com/OpenSIPS/opensips/commit/d2b911ff53cc226ec657e3f73f1ff3727c1542fe
  Author: Razvan Crainea <razvan at opensips.org>
  Date:   2023-03-07 (Tue, 07 Mar 2023)

  Changed paths:
    M modules/avpops/doc/avpops_admin.xml

  Log Message:
  -----------
  avpops: document the returned value in case of NULL

Due to the fact that an AVP cannot contain `NULL` values, we cannot
store `NULL` database values in AVPs. Hence, we store the `<null>` value
This commit simply documents the behavior, as reported in #3027

(cherry picked from commit fc35f7308efd6d2623e36adf048911be86b7b749)