io/ioutil: use pathname instead of name in docs to avoid confusion

io/ioutil: use pathname instead of name in docs to avoid confusion
caller of ioutil.TempFile() can use f.Name() to get "pathname"
of the temporary file, instead of just the "name" of the file.

Also remove an out-of-date comment about random number state.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6649054