Document extra account_types

The action android.settings.ADD_ACCOUNT_SETTINGS supports an extra
"account_types" (See AddAccountSettings & ChooseAccountActivity) but
that's not documented anywhere.
Oddly, the other 2 places that support the "authorities" extra do not
also support "account_types".

Change-Id: I99b7be0b0d6fff1cbccdf8b8b8e4fd2be9548e6c