add [diff] section to hgrc documentation

doc/hgrc.5.txt

@@ -151,6 +151,22 b' defaults::'
   defining command defaults. The command defaults will also be
   applied to the aliases of the commands defined.
 
+diff::
+  Settings used when displaying diffs. They are all boolean and
+  defaults to False.
+  git;;
+    Use git extended diff format.
+  nodates;;
+    Don't include dates in diff headers.
+  showfunc;;
+    Show which function each change is in.
+  ignorews;;
+    Ignore white space when comparing lines.
+  ignorewsamount;;
+    Ignore changes in the amount of white space.
+  ignoreblanklines;;
+    Ignore changes whose lines are all blank.
+
 email::
   Settings for extensions that send email messages.
   from;;