document the 'quit' controller command

svn:r4776

doc/control-spec.txt

@@ -467,6 +467,11 @@ $Id$
   Other flags may be defined in the future; Tor SHOULD ignore unrecognized
   flags.
 
+3.15 QUIT
+
+  Tells the server to hang up on this controller connection. This command
+  can be used before authenticating.
+
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