doc: Split protocol description paragraphs properly

The xsl translation from the protocol xml to publican would create only
one paragraph for all the text in a description. Make it generate one
paragraph for each block of text separated by two consecutive line
breaks instead.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>