Documentation: describe how to "bisect skip" a range of commits

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>

git-bisect.sh

@@ -9,7 +9,7 @@ git bisect bad [<rev>]
         mark <rev> a known-bad revision.
 git bisect good [<rev>...]
         mark <rev>... known-good revisions.
-git bisect skip [<rev>...]
+git bisect skip [(<rev>|<range>)...]
         mark <rev>... untestable revisions.
 git bisect next
         find next bisection to test and check it out.