doc: Replaced FakeNode with DocNode

doc: Replaced FakeNode with DocNode

The name FakeNode was a bad choice. It was used
to represent something that wasn't derived from
a C++ declaration in a .h file. Any generic page
or QML item or any special kind of qdoc construct
was stored in a FakeNode. The name was unfortunate
because the constructs stored in FakeNodes were
just as real as C++ constructs.

So FakeNode has been renamed to DocNode, which
just refers to a documentation node. The node
type Fake has been replaced with node type
Document.

Change-Id: Ida9de8288b7b8915bf9273fd890ca84aaf05e182
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>