os: clearer doc for Interrupt

I was confused by the juxtaposition of os.Interrupt docs, which are
"guaranteed to exist on all platforms" in one sentence and then
"not implemented" in the next sentence. Reading the code reveals
"not implemented" refers specifically to the implementation of
os.Process.Signal on Windows, not to the os.Interrupt variable itself.
Reword the doc to make this distinction clearer.

Fixes #27854.

Change-Id: I5fe7cddea61fa1954cef2006dc51b8fa8ece4d6e
Reviewed-on: https://go-review.googlesource.com/c/137336


Reviewed-by: Alex Brainman <alex.brainman@gmail.com>