MusicXML: convert <miscellaneous-field name='description'> tag to texidoc header

This allows the regtest cases to contain a verbose description of the
purpose of the test by simply placing it inside the
<identity>
  <miscellaneous>
	  <miscellaneous-field name='description'>
		   HERE COMES THE TEXIDOC STRING FOR LILYPOND (or any other application)!!!
	  </miscellaneous-field name='description'>
  </miscellaneous>
</identity>

Added descriptions to all regtest cases