Update HTML Modules explainer phrasing around allowed element types

[trusktr]

I modified

<script> elements of type="module" (non-module scripts will cause HTML Module creation to fail)

to

<script> elements of type="module" (non-module scripts will cause HTML Module creation to fail) and <template> elements

Is this correct? It seems to be what the explainer hints at.

[trusktr]

Nevermind, I was confused. At first I thought the original phrase meant only a <script> tag and no other tags could be in the HTML Module (which would be fairly useless). But I now saw other examples with arbitrary DOM elements besides the <script>.

New pull request: #880