New understanding text for 1.3.4 Identify Input Purpose

Have reworked the text of "Understanding 1.3.4 Identify Input Purpose", including substantial rewording kindly provided by Patrick. 

https://github.com/w3c/wcag21/pull/890 <https://github.com/w3c/wcag21/pull/890>

The Sufficient Techniques section has but one Technique so far. Not sure if other ways of adding programmatically determinable purpose are mature enough to be included at this point. I am uncertain whether the potential to furnish icons based on the autocomplete attribute should be played up here, or fit better into SC 1.3.5.
As to the Resources section, I would ask folks from the COGA TF to suggest sufficiently stable and relevant pointers.