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/
if ENABLE_DOC
subdir('reference')
endif
if ENABLE_GTK
pidgin_man = configure_file(
input : 'pidgin3.1.in',
output : 'pidgin3.1',
configuration : man_conf,
install : true,
install_dir : get_option('mandir') + '/man1')
endif
if enable_consoleui
finch_man = configure_file(
input : 'finch3.1.in',
output : 'finch3.1',
configuration : man_conf,
install : true,
install_dir : get_option('mandir') + '/man1')
endif