blame-options.txt: Add default value for -M/-C options.

Both `-M` and `-C` have default values and the <num> argument
the last `-C` option takes effect.

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/blame-options.txt

@@ -90,9 +90,9 @@ of lines before or after the line given by <start>.
 	running extra passes of inspection.
 +
 <num> is optional but it is the lower bound on the number of
-alphanumeric characters that git must detect as moving
+alphanumeric characters that git must detect as moving/copying
 within a file for it to associate those lines with the parent
-commit.
+commit. The default value is 20.
 
 -C|<num>|::
 	In addition to `-M`, detect lines moved or copied from other
@@ -105,9 +105,11 @@ commit.
 	looks for copies from other files in any commit.
 +
 <num> is optional but it is the lower bound on the number of
-alphanumeric characters that git must detect as moving
+alphanumeric characters that git must detect as moving/copying
 between files for it to associate those lines with the parent
-commit.
+commit. And the default value is 40. If there are more than one
+`-C` options given, the <num> argument of the last `-C` will
+take effect.
 
 -h::
 --help::