The SetFinalizer documentation states that

"The argument obj must be a pointer to an object allocated by calling
new or by taking the address of a composite literal."

which precludes pointers to local variables. According to a comment
on #6591, this case is expected to work. This CL updates the documentation
for SetFinalizer accordingly.

Fixes #6591

Change-Id: Id861b3436bc1c9521361ea2d51c1ce74a121c1af
Reviewed-on: https://go-review.googlesource.com/29592
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>