docs/config: give a relative includeIf example

The changes in the previous commit hopefully clarify that
the evaluation of an include "path" variable is the same no
matter if it's in a conditional section or not. But since
this question came up on the list, let's add an example that
makes it obvious.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -187,6 +187,12 @@ Example
 	[includeIf "gitdir:~/to/group/"]
 		path = /path/to/foo.inc
 
+	; relative paths are always relative to the including
+	; file (if the condition is true); their location is not
+	; affected by the condition
+	[includeIf "gitdir:/path/to/group/"]
+		path = foo.inc
+
 Values
 ~~~~~~