pidgin/pidgin

Add some missed docs for PidginCredential*

2021-05-13, Elliott Sales de Andrade
7e8c50c967bc
Add some missed docs for PidginCredential*

* Add `Since` tags for `PidginCredential`.
* Add docs for `PidginCredential*` top-level API.

Testing Done:
Compile

Reviewed at https://reviews.imfreedom.org/r/651/
IRC_SOURCES = [
'cmds.c',
'dcc_send.c',
'irc.c',
'irc.h',
'msgs.c',
'parse.c'
]
if DYNAMIC_IRC
irc_prpl = shared_library('irc', IRC_SOURCES,
dependencies : [sasl, libpurple_dep, glib, gio, ws2_32],
install : true, install_dir : PURPLE_PLUGINDIR)
endif