Avoid uninformative DESCRIPTION elements

[mbtaylor]

Including descriptive DESCRIPTION elements is good practice, but ones that just contain boilerplate are not much use.

Sec 3.2 says:

The TABLE must include:

• A human-readable DESCRIPTION that ideally conveys enough human-readable information that basic scientific use of the time series is possible without referring to literature.

I don't think this should be a MUST: authors who have no good description will just have to write something uninformative in here, which is not helpful. So I'd make that a SHOULD.

On a similar topic, the DESCRIPTIONs in the examples in sections 3.1 (photcal_PARAM.xml) and 4.1 (vot-ex1-GROUP.xml) read:

<DESCRIPTION>Photometric system description</DESCRIPTION>

which doesn't say anything additional to the name="PHOTCAL" label. Can something more informative be put in there?