Clean up and document sms_send hooks

Similarly to the hook cleanup in #2709473: Clarify the difference between sms_incoming operations

• Remove the $op param from existing process hooks, and separate into different hooks. Rename hooks so they are in line/opposite-of incoming hooks?
• Document the hooks in sms.api.php
• Clean up or standardise protected methods:
  1. merge into provider::send(): Merge provider::process, provider::preproces, provider::postprocess into send method? Or,
  2. add send terminology: rename them so they have 'send'/'outgoing' in the name. Because incoming has mirroring functionality, but without separate hooks. Or,
  3. Add separate protected methods to incoming, to mirror existing send methods? (preProcessIncoming... etc)