clarify -M without % symbol in diff-options

Signed-off-by: Sitaram Chamarty <sitaramc@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: sitaramc

Documentation/diff-options.txt

@@ -308,7 +308,11 @@ endif::git-log[]
 	index (i.e. amount of addition/deletions compared to the
 	file's size). For example, `-M90%` means git should consider a
 	delete/add pair to be a rename if more than 90% of the file
-	hasn't changed.
+	hasn't changed.  Without a `%` sign, the number is to be read as
+	a fraction, with a decimal point before it.  I.e., `-M5` becomes
+	0.5, and is thus the same as `-M50%`.  Similarly, `-M05` is
+	the same as `-M5%`.  To limit detection to exact renames, use
+	`-M100%`.
 
 -C[<n>]::
 --find-copies[=<n>]::