Use the "Core library -> runtime" section for changes that affect the
runtime package API and use the top-level "Runtime" section for
package-independent behavior changes. Also, move the one change that's
really about os (and net) into the "os" package section and reword it
to be more accurate.
Updates #37419.
Change-Id: I32896b039f29ac67308badd0d0b36e8c6e39f64f
Reviewed-on: https://go-review.googlesource.com/c/go/+/236718 Reviewed-by: Michael Knyszek <mknyszek@google.com>