Document "guard" flag. Also document subtle shift in meaning of "stable" and "fast" to mean "good for stable circs" and "good for fast circs", rather than "stable server" or "fast server".

svn:r5857

doc/dir-spec.txt

@@ -168,10 +168,12 @@ $Id$
           "Authority" if the router is a directory authority.
           "Exit" if the router is useful for building general-purpose exit
              circuits.
-          "Fast" if the router has high bandwidth.
+          "Fast" if the router is suitable for high-bandwidth circuits.
+          "Gurad" if the router is suitable for use as an entry guard.
+             (Currently, this means 'fast' and 'stable'.)
           "Named" if the router's identity-nickname mapping is canonical,
              and this authority binds names.
-          "Stable" if the router tends to stay up for a long time.
+          "Stable" if the router is suitable for long-lived circuits.
           "Running" if the router is currently usable.
           "Valid" if the router has been 'validated'.
           "V2Dir" if the router implements this protocol.