add GTK and CLI interfaces API documentation

doc/api.rst

 Monkeysign API documentation
 ============================
 
+GnuPG API
+---------
+
 .. automodule:: monkeysign.gpg
    :members:
+   :undoc-members:
+
+GTK Interface
+-------------
+
+.. automodule:: monkeysign.gtkui
+   :members:
+   :undoc-members:
+
+CLI Interface
+-------------
+
+.. automodule:: monkeysign.cli
+   :members:
+   :undoc-members: