pidgin/pidgin

Fix the docs that were broken when the sound api was removed

Update the docs for the sound api removal

Testing Done:
Built libpurple-doc, finch-doc, pidgin-doc, and facebook-doc, verified no errors.

Reviewed at https://reviews.imfreedom.org/r/68/
nullclient = executable('nullclient', 'nullclient.c', 'defines.h',
c_args : ['-DSTANDALONE'],
dependencies : [libpurple_dep, glib])