show new way for accessing pr test site credentials

Much more could be done but this is probably worth doing as a light edit

I didn't add in the ESR as I don't believe the policy  is clear enough to be documented but generally I understand that the discussion takes place on the ESR channel & PRs are not really encouraged against the branch - the CT would do a PR if they decide to backport or agree someone else would

Clarify that the ContactID refers to the user making the query - my reading had been that it was the contact ID of the contact that was being considered.

Explain what the `$type` parameter means/relates to.

Formatting tweak

I believe the getMenuKeyMax function is missing it's underscore inside the hook.

- Updated incorrect interface method name.
- Reorganized to match the de facto standard apparent on other hook pages.
- General content improvements.

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