doc: clarify how -S works

The existing text was very vague about what exactly it means
for difference to "contain" a change. This seems to cause
confusion on the mailing list every month or two.

To fix it we:

  1. use "introduce or remove an instance of" instead of
     "contain"

  2. point the user to gitdiffcore(7), which contains a more
     complete explanation

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -176,7 +176,10 @@ override configuration settings.
 	number.
 
 -S<string>::
-	Look for differences that contain the change in <string>.
+	Look for differences that introduce or remove an instance of
+	<string>. Note that this is different than the string simply
+	appearing in diff output; see the 'pickaxe' entry in
+	linkgit:gitdiffcore[7] for more details.
 
 --pickaxe-all::
 	When -S finds a change, show all the changes in that