[ANN] building ODK documentation now requires doxygen 1.8.4

[Michael Stahl]

forgot to add something about the comment syntax:

some people may be familiar with the autodoc comment syntax to get
hyperlinks, which is handled differently (and much simpler) in doxygen.

autodoc required types etc. to be annotated by XML tags like these,
which are not understood by doxygen and would lead to warnings and the
XML tags being copied literally to the output when used:

- <type>, <member>, <const>:
  stuff like that can simply be converted to a :: separated identifier,
  basically just like in the IDL syntax:

  * <type scope="bar">Foo</type> becomes
     bar::Foo
   (it's not a problem to have a single "." or ":" following that
   without a space between if needed for proper grammar)

  * members that are methods <member>method</member> need to have "()"
    suffix:
     method()

  * special case <member>Foo</member> (without any scope) needs to be
    prefixed with "#" to get recognized as a link:
     #Foo

- <TRUE/> <FALSE/> <VOID/> <NULL/> <true/> <false/> <void/> <null/>
  don't exist in doxygen, just use something like `TRUE` etc.

- <atom>42</atom> can be replaced with 42

- <arg>foo</arg> can be replaced with foo

- <method> and <module> were used in the IDL files but actually autodoc
  didn't understand them at all