enhance DefaultSettings.php documentation

enhance DefaultSettings.php documentation

This tweak our doxygen documentation for the DefaultSettings.php. Some
paragraphs were simply reformatted, some received additions.

To nicely render examples, I used the following snippet:

  @par Example:
  @code
  // some code there
  @endcode

That replaces all the <code> occurences.

Replaced some notes and warnings with @note and @warning to output a
nicely formatted box in our HTML documentation.

Change-Id: Iea8d4e46eac2d971af43d47499d121d0c4e2bff3