[docs] Add missing documentation for selectgroups for selects with optgroups

[brendanheywood]

What is the Moodle feature that needs documenting?

selectgroups for selects with optgroups has existed for a while and is undocumented.

Is this documentation specific to a Moodle version?

None

Are you able to provide a patch for this?

None

[hamidayubdev]

@sarjona I want to work on this missing documentation. Can you do me a favor of assigning it to me?

[sarjona]

Hi @hamidayubdev!
That's great news! I've just assigned you to this issue! :-)
In this page you'll find information about the process to install and prepare a patch for devdocs: https://moodledev.io/general/documentation/contributing

Please, let us know if there is anything you need from us ;-)