docs: Add version information to AGI command XML elements.

docs: Add version information to AGI command XML elements.

This process was a bit different than the others because everything
is in the same file, there's an array that contains the command
names and their handler functions, and the last command was created
over 15 years ago.

* Dump a `git blame` of res/res_agi.c from BEFORE the handle_* prototypes
  were changed.
* Create a command <> handler function xref by parsing the the agi_command
  array.
* For each entry, grep the function definition line "static int handle_*"
  from the git blame output and capture the commit.  This will be the
  commit the command was created in.
* Do a `git tag --contains <commit> | sort -V | head -1` to get the
  tag the function was created in.
* Add a single since/version element to the command XML.  Multiple versions
  aren't supported here because the branching and tagging scheme changed
  several times in the 2000's.