Bisect: add "bisect skip" to the documentation.

Also fix "bisect bad" and "bisect good" short usage description.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-bisect.txt

@@ -16,8 +16,9 @@ The command takes various subcommands, and different options depending
 on the subcommand:
 
  git bisect start [<bad> [<good>...]] [--] [<paths>...]
- git bisect bad <rev>
- git bisect good <rev>
+ git bisect bad [<rev>]
+ git bisect good [<rev>...]
+ git bisect skip [<rev>...]
  git bisect reset [<branch>]
  git bisect visualize
  git bisect replay <logfile>
@@ -134,6 +135,20 @@ $ git reset --hard HEAD~3		# try 3 revs before what
 Then compile and test the one you chose to try. After that, tell
 bisect what the result was as usual.
 
+Bisect skip
+~~~~~~~~~~~~
+
+Instead of choosing by yourself a nearby commit, you may just want git
+to do it for you using:
+
+------------
+$ git bisect skip                 # Current version cannot be tested
+------------
+
+But computing the commit to test may be slower afterwards and git may
+eventually not be able to tell the first bad among a bad and one or
+more "skip"ped commits.
+
 Cutting down bisection by giving more parameters to bisect start
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~