text/template: more godoc links

text/template: more godoc links

Change-Id: I4f3684ea6cbdcc09985c93fef5107d3bd006699e
Reviewed-on: https://go-review.googlesource.com/c/go/+/544735
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>