Correct return types.

Correct return types both in doc blocks and code if possible.

For methods like etag() I would rather like to simplify the return type from `@return string|null` to just "string" for example, though. Especially where there cannot be any empty valid strings or '0' strings involved.
:+1:  because it adds verbosity and thus makes things more. Also it makes my IDE happy and this means I don't get distracting notification colors throughout the code.

Last but not least: ``void`` is not a return (data) value but only the hint that you don't have to expect this code to return anything (not that it potentially still could, e.g. ``null`` by default).