cmd/cgo: documentation for C.GoString could be more complete

Documentation for C.GoString(), C.GoStringN(), and C.GoBytes() should probably explicitly document behaviour for "special" values (e.g. NULL pointer, negative integers).

https://golang.org/cmd/cgo/#hdr-Go_references_to_C

System details

go version go1.12.6 linux/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/nschelle/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/nschelle/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
GOROOT/bin/go version: go version go1.12.6 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.12.6
uname -sr: Linux 4.4.0-146-generic
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.6 LTS
Release:	16.04
Codename:	xenial
/gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.28.
gdb --version: GNU gdb (GDB) 8.3