[CLEANUP] Cleanup phpDoc and namespace usage in ext:frontend/ContentObject

Resolves: #62769
Releases: master
Change-Id: If5b0cde8d9d393b498230d3ee87a0f0bddaf9bb2
Reviewed-on: http://review.typo3.org/33866
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>