cmd/compile: rename retsigerr to sigerr

cmd/compile: rename retsigerr to sigerr

retsigerr was used to create error message for both wrong function
arguments and return arguments, so change its name to sigerr to reflect
that.

While at it, also add documentation for the wrong function arguments
case.

Passes toolstash-check.

Change-Id: I740c717ad38d4afab9e8c20f2e94579c8bca67ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/255240
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>