Warn against risk every time we give the email address.

wiki/src/doc/about/openpgp_keys/signing_key_revocation.mdwn

@@ -277,6 +277,9 @@ in case you want to assemble the revocation certificate is:
 
     address@example.org
 
+Do not write to this mailing list unless you really want to start the
+key revocation process.
+
 We are also copying below a summary of the mechanism.
 
 XXX: Copy the invitation email:
@@ -311,5 +314,8 @@ want to assemble the revocation certificate is:
 
     address@example.org
 
+Do not write to this mailing list unless you really want to start the
+key revocation process.
+
 Thanks, and may the force be with you!
 </pre>