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/
#define PIDGIN_ICON 104
#define PIDGIN_TRAY_AVAILABLE_4BIT 105
#define PIDGIN_TRAY_AWAY_4BIT 106
#define PIDGIN_TRAY_BUSY_4BIT 107
#define PIDGIN_TRAY_XA_4BIT 108
#define PIDGIN_TRAY_OFFLINE_4BIT 109
#define PIDGIN_TRAY_CONNECTING_4BIT 110
#define PIDGIN_TRAY_PENDING_4BIT 111
#define PIDGIN_TRAY_INVISIBLE_4BIT 112