Expand QML_EXTENDED_NAMSPACE's documentation

Note that it doesn't necessarily need a namespace; mention that it
however requires the "namespace" and its contained enums to be exposed
to the metaobject system to work.
Moreover, add a minimal code example.

Change-Id: I814c65b2b59747f902f2291e2f98962eb98d4d59
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
(cherry picked from commit a3959ea9df49b5c30833d49daede5bf60a70897e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

0 files changed, 0 insertions, 0 deletions