description strings should be reworded

[prjemian]

As discussed at NIAC2018, the final wording for <definitions>/nxdlTypes.xsd in the documentation of <xs:simpleType name="NX_CHAR"> is:

The preferred string representation is UTF-8. 
Both fixed-length strings and variable-length strings are valid. 
String arrays cannot be used where only a string is expected 
(title, start_time, end_time, NX_class attribute,...). 
Fields or attributes requiring the use of string arrays will be 
clearly marked as such (like the NXdata attribute auxiliary_signals).

[prjemian]

also appears in manual/source/datarules.rst