docwriter/gjs: format GObject.Value as "Any"

GValues are transparently marshalled by GJS, so you can
never use them as boxed (and they are not part of the
documentation as such).
docwriter: format bitfields like enums

They are enums after all.
docwriter/gjs: format GObject.Closure as "Function"

GClosures are represented in gjs as JS functions for in
parameters, so it makes sense to use Function in place of
the boxed structure (which is also not part of the documentation)

Note that GClosures returned from C code are marshalled
as boxed types, but this is a gjs bug really, and JS code
should never touch them.
docwriter/gjs: restore documentation for return values
docwriter/gjs: fix boolean return values

'ok' != 'return_value', and we never restore 'Returns:'.

At the same time, do the same fix for (out) parameters if the
return value is void, so this

void my_object_get_foo (MyObject *object, Foo *)

is turned into

function get_foo() : Foo
Returns: ....
doctool: update tests