[DAS2] sources.rnc

[Ed]

I have nothing substantial to change in sources.rnc.  Just clean up the
notes and comments:

** Can this note be improved?

  # NOTE: the segments capability has optional 'coordinates'
  # element to state that it implements the given coordinate
  # system.  I could not figure out how to do that in Relax-NG.
##attribute coordinates { text },

** Several references to "At present...." should be removed

** Can this note be cleaned-up?  Which Andreas?  Is this the
** full list of reserved words?

# 'Chromosome', 'Clone', 'Contig', 'Scaffold', etc.
# This is from a restricted vocabulary maintained by Andreas
  attribute source { text },