cmd/compile: add documentation for isfat

In CL 192980, I tend to think that canSSAType can be used as replacement
for isfat. It is not the truth as @khr points me out that isfat has very
different purpose.

So this CL adds documentation for isfat, also remove outdated TODO.

Change-Id: I15954d638759bd9f6b28a6aa04c1a51129d9ae7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/196499


Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>