[IMP] adapt doc to the removal of voip_onsip

[oomsveta]

Task-5176220.

Enterprise: https://github.com/odoo/enterprise/pull/97465
Upgrade: https://github.com/odoo/upgrade/pull/8679

[robodoo]

[Felicious]

Appreciate you for keeping the documentation updated with your code change, @oomsveta !

For context, @jero-odoo, the OnSIP integration is made more direct in master, as we no longer need a separate module, and instead the Auth username just needs to match their username. (see line 77 of the doc). Can you double-check whether all the wording changes that oomsveta made are written clearly, have enough context for the everyday user, and everything is formatted according to our standards?

After that, we'll merge this PR the same time the enterprise PR merges!