doc/go1.14: move "Minor changes to the library" heading up
This heading was below the minor changes to the standard library.
It should be on top.
Many of the minor changes have been documented, so remove the broad
TODO comment. It is still a TODO to highlight more prominent changes
to the library, if there are any, under the "Core library" heading.
Updates #36878
Change-Id: If7fd9af9a933af917523e33fd2922c5f3c02c98b
Reviewed-on: https://go-review.googlesource.com/c/go/+/217277 Reviewed-by: Alexander Rakoczy <alex@golang.org> Reviewed-by: Carlos Amedee <carlos@golang.org>