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>