Make conduit-uri info easier to be copied into arcrc

Summary:
add the conduit URI and the username together with the arc
certificate to the setting page.

Test Plan:
run arc diff to make sure it still works after copying the
generated test into the .arcrc file.

Reviewed By: epriestley
Reviewers: epriestley
CC: epriestley
Differential Revision: 73

src/docs/arcanist_certificates.diviner

@@ -22,5 +22,4 @@ To retrieve the certificate for a Phabricator host:
   - Login to Phabricator.
   - Go to Settings > Arcanist Certificate.
 
-Copy the certificate into the "cert" field, and specify your username in the
-"user" field.
+Copy the host info including the certificate into the "hosts" section.