html/template: document handling of namespaced and data- attributes

Attributes with a namespace or a data- prefix are handled as if they
had no namespace/data- prefix.

There is also a special case, where attributes with a "xmlns" namespace
are always treated as containing URLs.

This could surprise users of the package, since this behaviour was not
documented anywhere, so this change adds some documentation for all
three cases.

Fixes #12648

Change-Id: If57a2ec49fec91a330fc04795726e8cffa9b75c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/79895


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>