SUPPORT.md, support matrix: Treat commentary before status as description

SUPPORT.md, support matrix: Treat commentary before status as description

Running text in feature sections in the markdown document currently
might be (i) a caveat, qualifying or clarifying the support statement
(ii) a plain description of the feature.

Caveats can be version-specific and deserve the [*] annotation in the
relevant feature matrix cell.  They must link to SUPPORT.html for the
specific version.

Descriptions are not version specific.  In that case the [*]
annotation is visusal noise.  Rather, it is better to make a hyperlink
out of the text which is being expanded on.  The hyperlink can point
to any appropriate version.

There is a question about how to notate this distinction in
SUPPORT.md.  After IRL discussion with George and Lars I propose that
we should put text which helps describe a feature (ie, which expands
on a section heading) after the heading but before the Status
indications; whereas, caveats and supplementary information about
the actual status, should follow the Status block.

This patch implements this distinction in the support matrix
generator.  Only paragraphs containing _only_ italic content count as
descriptive; anything else is treated as a caveat.

In the code:

 * Add a new entry to RealSect, HasDescription

 * When parsing, track whether we are before or after the first Status
   block in a new variable $has_feature.

 * In ri_Para, set HasDescription set to the input document index
   when we encounter text before the first feature.

 * When writing a `heading' (ie, the table cell for a feature name)
   look for HasDescription and make an appropriate hyperlink.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>