more prominently document existing scheme for uncertainties

[zjttoefs]

There is a _fieldname__errors way of adding uncertainties to arbitrary fields defined at least within NXdata. That could be generalised, but should be more widely known anyway.

[prjemian]

It was decided at NIAC2010 that the uncertainties attribute be added to all fields, by adding it under fieldType in nxdl.xsd

Show a simple example from https://github.com/canSAS-org/NXcanSAS_examples

[prjemian]

copy (and modify) definition of @uncertainties from #516

[prjemian]

done with #685