The syscall execve has no wrapper in this library, which it seems like
it should - Ian seemed to concur in CL 72550.

Add a docstring and an example, because I always get confused about
how to invoke the syscall.

Change-Id: I6100bbbf4ace9e3e341bf186a04cc03301da9aea
Reviewed-on: https://go-review.googlesource.com/101282Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>