res_agi: Improve documentation

* Groups of AGI commands that have similar functionality now reference
  each other, and all reference the AGI application for ease of wiki
  reference.

* The documentation for the AGI application has been improved, in
  particular noting the various AGI types and how they are invoked.

* A warning message has been added to DeadAGI, noting that it is
  deprecated.

Change-Id: I479ccdee8a7393f01b18692c3d4ab7e6bdd1875d