Render newlines as separate paragraphs

[praetorian20]

Line breaks in Doxygen documentation are not being rendered correctly.
Given the following C++ class definition

    /*!
     * @brief Lorem ipsum dolor sit amet
     */
    class foo
    {
    public:
       /* Fields */
       uint8_t   bar; /*!< @brief consectetur adipiscing elit
 	<br>Range: 0 .. 10<br>Default: 0*/
    }; // class foo

The corresponding XML output for the bar data member is

    <memberdef kind="variable" id="classfoo_1a1c39cf8cc2d68f0a7bf308aa34c3b5ba" prot="public" static="no" mutable="no">
      <type>uint8_t</type>
      <definition>uint8_t foo::bar</definition>
      <argsstring></argsstring>
      <name>bar</name>
      <briefdescription>
 <para>consectetur adipiscing elit <linebreak/>
 Range: 0 .. 10<linebreak/>
 Default: 0 </para>        </briefdescription>
      <detaileddescription>
      </detaileddescription>
      <inbodydescription>
      </inbodydescription>
      <location file="test.h" line="94" bodyfile="test.h" bodystart="93" bodyend="-1"/>
    </memberdef>

The <linebreak/> tags, which appear as newline characters in the node
argument's text in the call to SphinxRenderer.visit_unicode, do not get
rendered as paragraphs. The corresponding Sphinx HTML is

    <dd><p>consectetur adipiscing elit
    Range: 0 .. 10
    Default: 0 </p>
    </dd></dl>

This commit wraps each line in multiline comments in a paragraph so it gets
rendered correctly. After applying this change, the Sphinx HTML is rendered as
follows

    <dd><p>consectetur adipiscing elit <p>Range: 0 .. 10</p>
    <p>Default: 0 </p>
    </p>
    </dd></dl>

[vermeeren]

Thanks for the very detailed commit message/info, reminds me of kernel.org commits. :)

Will release 4.9.0 with this in a bit.