Edge supports WebAssembly but not TextEncoder or TextDecoder.
This change adds a comment pointing to a polyfill that could
be used. The polyfill is not added by default, because we want to
let the user decide if/how to include the polyfill.

Fixes #27295

Change-Id: I375f58f2168665f549997b368428c398dfbbca1c
Reviewed-on: https://go-review.googlesource.com/139037Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>