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

Add a note to say that this hook will only be invoked for API calls if check_permissions is set to 1

See https://lab.civicrm.org/dev/core/raw/master/CRM/Contact/BAO/Contact.php line 1177

See the https://lab.civicrm.org/dev/core/raw/master/CRM/Contact/BAO/Contact.php line 1188

Clarified when the backtrace is displayed, i.e. calling &backtrace=1 does not trigger an immediate backtrace unless an error occurs.

The phrase originated in one context (i.e.  discussing a change in the
version-numbering pattern).  In the context of `info.xml` docs, it seemed
helpful to use more concrete wording ("Because 4.7.x and 5.x are
substantively the same series...").

Updating docs to reflect changes made here: dev/release#1 (comment 2843)