gitdiffcore doc: update pickaxe description

The old text described the original design (one side does not have it at
all while the other side has it); this was later amended to check if the
number of occurrences changed, which is what we currently do with -S.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitdiffcore.txt

@@ -227,9 +227,9 @@ changes that touch a specified string, and is controlled by the
 commands.
 
 When diffcore-pickaxe is in use, it checks if there are
-filepairs whose "result" side has the specified string and
-whose "origin" side does not.  Such a filepair represents "the
-string appeared in this changeset".  It also checks for the
+filepairs whose "result" side and whose "origin" side have
+different number of specified string.  Such a filepair represents
+"the string appeared in this changeset".  It also checks for the
 opposite case that loses the specified string.
 
 When `\--pickaxe-all` is not in effect, diffcore-pickaxe leaves