doc/go1.14: mention better error checking in text/template
    
    This caused 35 test failures in Google internal code,
    so it's worth mentioning in the release notes.
    
    Updates #31810
    Updates #36878
    Fixes #37066
    
    Change-Id: I2faa6bce4c7d735107eceaef7d95223844846454
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217978
    Reviewed-by: Rob Pike <r@golang.org>