Document QSGTexture.
Document QSGTexture.

Task-number: QTBUG-23192
Change-Id: I4a91dfaa0aa80d3cdc6c785fcaefdeafe60a2149
Reviewed-by: Gunnar Sletta <d0b71651f3efaa9fda200bbcc395b7b82a871fdd@nokia.com>

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