Improve Markdown (and other) module doc output

- The html_ify filter now escapes HTML found in module documentation.
  THIS COULD AFFECT MORE THAN JUST MARKDOWN but I didn't see any modules
  expecting to use e.g. HTML entities or HTML tags in their
  documentation.

- The markdown_ify filter (used as jpfunc in markdown.j2) escapes at
  least a few Markdown in-line formatting characters.

- Improvements to markdown.j2:

    - Call jpfunc on the module name heading so that it gets escaped for
      Markdown (e.g. my_module_name becomes my\_module\_name).

    - Added paragraph breaks between paragraphs in the description.

    - Added examples heading, which is consistent with the notes heading
      below it.

hacking/templates/markdown.j2

@@ -1,4 +1,4 @@
-## @{ module }@
+## @{ module | jpfunc }@
 
 {# ------------------------------------------
  #
@@ -11,7 +11,8 @@ New in version @{ version_added }@.
 {% endif %}
 
 {% for desc in description -%}
-@{ desc | jpfunc }@ 
+@{ desc | jpfunc }@
+
 {% endfor %}
 
 {% if options -%}
@@ -35,6 +36,10 @@ New in version @{ version_added }@.
 </table>
 {% endif %}
 
+{% if examples or plainexamples %}
+#### Examples
+{% endif %}
+
 {% for example in examples %}
 {% if example['description'] %}
 * @{ example['description'] | jpfunc }@