Add docs for passwordeval function

docs/api.rst

@@ -15,6 +15,7 @@ General Section
     [general]
     status_path = ...
     #processes = 0
+    #passwordeval =
 
 
 - ``status_path``: A directory where vdirsyncer will store metadata for the
@@ -38,6 +39,10 @@ General Section
       to anything else than ``1`` will mean that you can't properly abort the
       program with ``^C`` anymore.
 
+- ``passwordeval`` specifies a command to query for server passwords. The
+  command will be called with the username as the first argument, and the
+  hostname as the second.
+
 .. _pair_config:
 
 Pair Section