acc: docs - database column constraint alteration for acc_extra_nullable and cdr_extra_nullable [skip ci]

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#### Checklist: <!-- Go over all points below, and after creating the PR, tick the checkboxes that apply --> - [ ] PR should be backported to stable branches - [ ] Tested changes locally - [X] Related to issue #2531 (replace XXXX with an open issue number)

#### Description <!-- Describe your changes in detail --> Documentation update related to https://github.com/kamailio/kamailio/issues/2531#issuecomment-724762135 You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/2550

-- Commit Summary --

* acc: docs - database column constraint alteration for acc_extra_nullable and cdr_extra_nullable [skip ci]

-- File Changes --

M src/modules/acc/doc/acc_admin.xml (8)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/2550.patch https://github.com/kamailio/kamailio/pull/2550.diff