Documentation: clarify diff --cc

The definition of an "uninteresting" hunk was not in line with reality.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-diff-tree.txt

@@ -93,11 +93,11 @@ include::pretty-options.txt[]
 	This flag changes the way a merge commit patch is displayed,
 	in a similar way to the '-c' option. It implies the '-c'
 	and '-p' options and further compresses the patch output
-	by omitting hunks that show differences from only one
-	parent, or show the same change from all but one parent
-	for an Octopus merge.  When this optimization makes all
-	hunks disappear, the commit itself and the commit log
-	message is not shown, just like in any other "empty diff" case.
+	by omitting uninteresting hunks whose the contents in the parents
+	have only two variants and the merge result picks one of them
+	without modification.  When all hunks are uninteresting, the commit
+	itself and the commit log message is not shown, just like in any other
+	"empty diff" case.
 
 --always::
 	Show the commit itself and the commit log message even

Documentation/rev-list-options.txt

@@ -108,9 +108,9 @@ options may be given. See linkgit:git-diff-files[1] for more options.
 --cc::
 
 	This flag implies the '-c' options and further compresses the
-	patch output by omitting hunks that show differences from only
-	one parent, or show the same change from all but one parent for
-	an Octopus merge.
+	patch output by omitting uninteresting hunks whose contents in
+	the parents have only two variants and the merge result picks
+	one of them without modification.
 
 -r::