21151: document datadir default decently.

changes/data_dir_default_doc

@@ -0,0 +1,3 @@
+  o Documentation:
+    - Correct the documentation about the default DataDirectory value.
+      Closes ticket 21151.

doc/tor.1.txt

@@ -394,7 +394,9 @@ GENERAL OPTIONS
 
 [[DataDirectory]] **DataDirectory** __DIR__::
     Store working data in DIR. Can not be changed while tor is running.
-    (Default: @LOCALSTATEDIR@/lib/tor)
+    (Default: ~/.tor if your home directory is not /; otherwise,
+    @LOCALSTATEDIR@/lib/tor.  On Windows, the default is
+    your ApplicationData folder.)
 
 [[DataDirectoryGroupReadable]] **DataDirectoryGroupReadable** **0**|**1**::
     If this option is set to 0, don't allow the filesystem group to read the