Document the exception behaviour of defer

[hanche]

The current documentation for defer says only this about exceptions:

any exception it throws gets propagated

Experimenting a bit reveals two undocumented facts:

• The deferred function is called even if the containing closure throws an exception. Then, the exception is rethrown. Thus, this provides an alternative to using try { … } finally { … } in some cases.
• If both the deferred function and the containing closure throw an exception, only the latter is propagated. Clearly, it makes no sense to propagate both, but at least the behaviour should be documented.

Examples:

~> { defer { put done }; fail foo }
▶ done
Exception: foo
Traceback:
  [tty 1]:1:23:
    { defer { put done }; fail foo }
  [tty 1]:1:1:
    { defer { put done }; fail foo }
~> { defer { put done; fail oops }; fail foo }
▶ done
Exception: foo
Traceback:
  [tty 2]:1:34:
    { defer { put done; fail oops }; fail foo }
  [tty 2]:1:1:
    { defer { put done; fail oops }; fail foo }

(I had somehow managed to miss defer until now, which is why I felt a need to play with it.)

[hanche]

One more defer behaviour that merits documentation: If a closure contains several defer calls, their functions will be called in the reverse order, from the last to the first one.