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/
subdir('bonjour')
subdir('facebook')
subdir('gg')
subdir('irc')
subdir('jabber')
subdir('novell')
subdir('null')
subdir('sametime')
subdir('silc')
subdir('zephyr')