docs/parse-support-md: Allow definition lists for features

docs/parse-support-md: Allow definition lists for features

Now, as well as a `code block', with
  |    Something: some status
we tolerate a definition list which in pandoc terms looks like this
  |Term
  |: Definition

This ought not usually be be used for features but it will be useful
for linking to the release notes, because markup is not allowed in
code blocks but is in definitions.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Juergen Gross <jgross@suse.com>