Disambiguate buildForm docs, add TODO

Xml::buildForm needs some love. I've given it a better documentation
string, but left a TODO for accessibility: We need to use labels for
input labelling, not table elements.

Change-Id: I4a9ff174e83c8ca3242c82bbe7cb0da0b8be7785
Merge "Disambiguate buildForm docs, add TODO"