Add in notes about support for KeyColumn parameter when using option group as part of a setting pseudoconstant

Note working through this threw up some ambiguity about help_text vs help which I'll dig into as a follow up. I think there is further text on the page I need to review too about the settings form

Following the discussion here: https://chat.civicrm.org/civicrm/pl/cfon7833xbyxpcjjcpqs1qqqja

Some more details about creating a settings page for an extension.

Typo

Changed the line numer in the link of the example file for declaring the settings folder.

Update framework/setting with updated link for settings override and also add in notes in regards to group_name now being domain or contact in 4.7 and update example