Documentation: boolean value may be given by on/off

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -61,7 +61,7 @@ Internal whitespace within a variable value is retained verbatim.
 
 The values following the equals sign in variable assign are all either
 a string, an integer, or a boolean.  Boolean values may be given as yes/no,
-0/1 or true/false.  Case is not significant in boolean values, when
+0/1, true/false or on/off.  Case is not significant in boolean values, when
 converting value to the canonical form using '--bool' type specifier;
 'git-config' will ensure that the output is "true" or "false".