1170 nxxas documentation improvement

[benajamin]

fixes #1170 with minimal changes.

[prjemian]

Suggest adding some versioning information, such as this from NXentry:

definitions/base_classes/NXentry.nxdl.xml

Lines 134 to 153 in 78141ca

	<field name="definition">
	<doc>
	(alternate use: see same field in :ref:`NXsubentry` for preferred)
	Official NeXus NXDL schema to which this entry conforms which must be
	the name of the NXDL file (case sensitive without the file extension)
	that the NXDL schema is defined in.
	For example the ``definition`` field for a file that conformed to the
	*NXarpes.nxdl.xml* definition must contain the string **NXarpes**.
	This field is provided so that :ref:`NXentry` can be the overlay position
	in a NeXus data file for an application definition and its
	set of groups, fields, and attributes.
	*It is advised* to use :ref:`NXsubentry`, instead, as the overlay position.
	</doc>
	<attribute name="version"><doc>NXDL version number</doc></attribute>
	<attribute name="URL"><doc>URL of NXDL file</doc></attribute>
	</field>

[prjemian]

See #1038 for question on versioning

[PeterC-DLS]

LGTM

[benajamin]

Proposal is to accept the changes to NXxas in pull request #1190

[padraic-shafer]

Hi, I'm late to the party here but wanted to say that these look like positive changes, and I'd like to find a way to keep this list of NXxas/DATA/modes up-to-date.

In the near term, we would like to see "Luminescence Yield" added as option. I think it would it would be helpful to include an "OTHER" such as "Other Transmissive" and "Other Absorptive" as additional expansion options.

In the next month or so, we'll be submitting a follow-up enhancement of the NXxas definition that supports multi-detector measurements. Simply wanted to add a placeholder before anything gets backward incompatible.

Suggestions are welcome!