doc/go1.18: add short-circuit evaluation for text/template

This description is based on https://golang.org/cl/321490.

Updates #47694

Change-Id: I48656cd487d2fccf0b0d3390f350f1bc6f2b0080
Reviewed-on: https://go-review.googlesource.com/c/go/+/365738


Trust: Jeremy Faller <jeremy@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>