add document on command defaults

doc/hgrc.5.txt

@@ -135,6 +135,21 b' decode/encode::'
     # them to the working dir
     **.txt = tempfile: unix2dos -n INFILE OUTFILE
 
+defaults::
+  Use the [defaults] section to define command defaults, i.e. the 
+  default options/arguments to pass to the specified commands.
+  
+  The following example makes 'hg log' run in verbose mode, and
+  'hg status' show only the modified files, by default.
+  
+    [defaults]
+    log = -v
+    status = -m
+  
+  The actual commands, instead of their aliases, must be used when 
+  defining command defaults. The command defaults will also be
+  applied to the aliases of the commands defined.
+
 email::
   Settings for extensions that send email messages.
   from;;