Document -B<n>[/<m>], -M<n> and -C<n> variants of -B, -M and -C

These options take an optional argument, but this optional argument was
not documented.

Original patch by Matthieu Moy, but documentation for -B mostly copied
from the explanations of Junio C Hamano.

While we're there, fix a typo in a comment in diffcore.h.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diffcore.h b/diffcore.h
index 491bea0..fed9b15 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -18,7 +18,7 @@
 #define MAX_SCORE 60000.0
 #define DEFAULT_RENAME_SCORE 30000 /* rename/copy similarity minimum (50%) */
 #define DEFAULT_BREAK_SCORE  30000 /* minimum for break to happen (50%) */
-#define DEFAULT_MERGE_SCORE  36000 /* maximum for break-merge to happen 60%) */
+#define DEFAULT_MERGE_SCORE  36000 /* maximum for break-merge to happen (60%) */
 
 #define MINIMUM_BREAK_SIZE     400 /* do not break a file smaller than this */