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>