DocBook: make Paragraph::simpleDocBookOnePar return a list of paragraphs.

DocBook: make Paragraph::simpleDocBookOnePar return a list of paragraphs.

Before, it directly wrote to the XMLStream, but it made implementation of new lines tricky. Now, it returns the XML for each sub-paragraph (delimited by new lines) as a string, so that the caller can adopt a more precise behaviour (such as in lists).