Formatting tweak

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

Describe new variables due to dev/core#457

Describe new variables due to dev/core#457

Adding this clause has led to unreliable query results - better to give no information than to lead them wrong - many reports now disable & reenable full group by but that is referred to elsewhere.

Also better to override whereClause & selectClause

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

dev/core#90 Add in documentation of the disabling and re-enabling of the ONLY_FULL_GROUP_BY sql mode for developers to use

Fix comma splice.  Make clearer reference to later section.

* Emphasize use-cases first -- and then drill-down to tools/specifics.
* Add a placeholder section for extension PRs
* Re-style and copy-edit the section for core PRs

based on https://chat.civicrm.org/civicrm/pl/etog75icwjywt84sxtw11fumoh