dsls: stop parsing documentation blocks at lines that are empty or contain only spaces

When given the following DSL-like code:

    # All properties follow

    # The timeout in seconds for reading I/O
    property('timeout', 'double')

The current code would have included 'All properties follow' in the
documentation for the timeout property. This seems very unintuitive, and IMO we
should stop at the blank line. If one wants to separate documentation
paragraphs, he should do the standard way of

  # All properties follow
  #
  # The timeout in seconds for reading I/O
  property('timeout', 'double')