clarify use of the symbols table in a NXDL file

[prjemian]

The symbols table documentation would benefit from more guidance, as suggested in the symbol - for any levels including 'root'? item of nexusformat/NIAC#107.

[prjemian]

Describe why it is at the top level of a NXDL file: to avoid name collision and document each symbol that is used

[prjemian]

Such as this table from h5py.

[prjemian]

This might be a good addition to the Symbols table section of defs_intro.rst:

Any symbol used in a NXDL file should be defined in the symbols table.

Also add to this documentation:

definitions/nxdl.xsd

Lines 201 to 209 in 61c2b2a

	<xs:element name="symbols" type="nx:symbolsType" minOccurs="0" maxOccurs="1">
	<xs:annotation>
	<xs:documentation>
	Use a ``symbols`` list
	to define each of the mnemonics that
	represent the length of each dimension in a vector or array.
	</xs:documentation>
	</xs:annotation>
	</xs:element>