Update hook_civicrm_export

Describe new variables due to dev/core#457

add 'CIVICRM_TEMP_FORCE_DURABLE'

Add in documentation of new hook hook_civicrm_postIPNProcess

Clarifications to buildkit path docs

Fix typo in hook_civicrm_links 

Remove unnecessary "are" in upgrade.md

Continuous Integration - Improve layout and add section for extension PRs

Cache Reference - Link to admin guide. Add section on custom caches.

Update hook_civicrm_pre.md

Update hook_civicrm_post.md

Update debugging.md

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

Update info on civix generate:api

CRM-20845: document alterMailingRecipients hook

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

 Update git.md - typo

Update hook_civicrm_selectWhereClause.md

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

Add CIVICRM_MAIL_LOG_AND_SEND

Include Backdrop in the top level list