Add documentation for the options

[infinity0]

It would be nice to explain the options in a bit more detail. For example:

• I don't know what the "default" keyring is. In the old mozilla-gnome-keyring, this was the "mozilla" keyring. Is it now the "login" keyring? But I also see "login" in the drop-down box for "use an available keyring", and there should be an indication that this is the same as the "default" keyring (if this is indeed true; I am not sure).
• What does "store passwords only for this session" mean? The browser session, or the desktop session?

[swick]

The default keyring is the the keyring marked as default by your system. Might be useful to display the name of the current default.

"store passwords only for this session" stores passwords only for the duration of the desktop session.