res_agi: Fix xmldocs bug with set music.

The XML documentation for the SET MUSIC AGI
command is invalid, as the parameter does not
have a name and the on/off enum options for
the on/off argument are listed separately, which
is incorrect. The cumulative effect of these currently
is that the Asterisk Wiki documentation for SET MUSIC
is broken and external documentation generators crash
on SET MUSIC due to the malformed documentation.

These issues are corrected so that the documentation
can be successfully parsed as with other similar AGI
commands.

ASTERISK-29939 #close
ASTERISK-28891 #close

Change-Id: I8c3d59897531bcbc401cbc7b00c9e2829dcb35f8