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/
libpurple/data/purple-url-handler.desktop.in
libpurple/protocols/null/nullprpl.c
pidgin/data/im.pidgin.Pidgin.desktop.in
sub/libpurple/data/purple-url-handler.desktop.in
sub/libpurple/protocols/null/nullprpl.c
sub/pidgin/data/im.pidgin.Pidgin.desktop.in