Document the default PathsNeededToBuildCircuits value

... when the directory authorities don't set min_paths_for_circs_pct.

Fixes bug 20117; bugfix on 02c32091 in tor-0.2.4.10-alpha.
Reported by Jesse V.

changes/bug20117

+  o Minor bugfixes (documentation):
+    - Document the default PathsNeededToBuildCircuits value that's
+      used by clients when the directory authorities don't set
+      min_paths_for_circs_pct.
+      Fixes bug 20117; bugfix on 02c320916e02 in tor-0.2.4.10-alpha.
+      Patch by teor, reported by Jesse V.

doc/tor.1.txt

@@ -1558,8 +1558,9 @@ The following options are useful only for clients (that is, if
     that fraction of possible paths. Note that setting this option too low
     can make your Tor client less anonymous, and setting it too high can
     prevent your Tor client from bootstrapping.  If this option is negative,
-    Tor will use a default value chosen by the directory
-    authorities. (Default: -1.)
+    Tor will use a default value chosen by the directory authorities. If the
+    directory authorities do not choose a value, Tor will default to 0.6.
+    (Default: -1.)
 
 [[ClientBootstrapConsensusAuthorityDownloadSchedule]] **ClientBootstrapConsensusAuthorityDownloadSchedule** __N__,__N__,__...__::
     Schedule for when clients should download consensuses from authorities