os: document that Exit does not run deferred calls

[campoy]

What steps will reproduce the problem?
1. Run this program http://play.golang.org/p/CL9yMyhXA_
2. The defer fmt.Println is never executed
3. Why? The spec says that defer will be executed at the end of the function.

What is the expected output? What do you see instead?
The behavior is good, but the documentation should mention this case.
runtimer.Goexit() which could have a similar behavior specifies that it calls all the
deferred calls. We should tell that os.Exit doesn't call them.

Please use labels and text to provide additional information.

[rsc]

Comment 1:

Labels changed: added priority-go1.1, removed priority-triage.

[rsc]

Comment 2:

Labels changed: added priority-later, go1.1, removed priority-go1.1.

[rsc]

Comment 3:

Labels changed: added size-s.

[adg]

Comment 4:

This issue was closed by revision 0f64a6e.

Status changed to Fixed.