text/template,html/template: state that Funcs must happen before parsing

text/template,html/template: state that Funcs must happen before parsing

Any method that affects the parse must happen before parsing.
This obvious point is clear, but it's not clear to some that the
set of defined functions affect the parse.

Fixes #18971

Change-Id: I8b7f8c8cf85b028c18e5ca3b9797de92ea910669
Reviewed-on: https://go-review.googlesource.com/38413
Reviewed-by: Ian Lance Taylor <iant@golang.org>