Normalise Tai Tham or not?

[Eli Zaretskii via Unicode]

> Date: Wed, 11 Oct 2017 22:01:32 +0100
> From: Richard Wordingham via Unicode <unicode at unicode.org>
> 
> The description I had found undersold the noble intention.

If you mean that the documentation doesn't describe the feature well
enough, I'd welcome a documentation bug report.

> Unfortunately, it doesn't work in general with unnormalised text.

Indeed, it wasn't supposed to, on the assumption that those are
relatively rare cases, and for an initial implementation could be left
unsupported.