tutorial: gentler illustration of Alice/Bob workflow using gitk

Update to gitutorial as discussedin the git mailing list:

http://marc.info/?t=121969390900002&r=1&w=2

Signed-off-by: Paolo Ciarrocchi <paolo.ciarrocchi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gittutorial.txt

@@ -321,10 +321,37 @@ pulling, like this:
 
 ------------------------------------------------
 alice$ git fetch /home/bob/myrepo master
-alice$ git log -p ..FETCH_HEAD
+alice$ git log -p HEAD..FETCH_HEAD
 ------------------------------------------------
 
 This operation is safe even if Alice has uncommitted local changes.
+The range notation HEAD..FETCH_HEAD" means "show everything that is reachable
+from the FETCH_HEAD but exclude anything that is reachable from HEAD.
+Alice already knows everything that leads to her current state (HEAD),
+and reviewing what Bob has in his state (FETCH_HEAD) that she has not
+seen with this command
+
+If Alice wants to visualize what Bob did since their histories forked
+she can issue the following command:
+
+------------------------------------------------
+$ gitk HEAD..FETCH_HEAD
+------------------------------------------------
+
+This uses the same two-dot range notation we saw earlier with 'git log'.
+
+Alice may want to view what both of them did since they forked.
+She can use three-dot form instead of the two-dot form:
+
+------------------------------------------------
+$ gitk HEAD...FETCH_HEAD
+------------------------------------------------
+
+This means "show everything that is reachable from either one, but
+exclude anything that is reachable from both of them".
+
+Please note that these range notation can be used with both gitk
+and "git log".
 
 After inspecting what Bob did, if there is nothing urgent, Alice may
 decide to continue working without pulling from Bob.  If Bob's history