clarify that format must be a string literal in OpenCL_C.txt

[aharon-abramson]

The intent behind "resolvable at compile time" is that format cannot be a kernel argument (see https://github.com/KhronosGroup/OpenCL-Docs/issues/1325#issuecomment-268542016)

[SunSerega]

I think the old text allowed a constant variable containing format, not just a string literal.

That way, we can define these formats at the top and reuse them for multiple printf calls without #define-s.

[CLAassistant]

All committers have signed the CLA.

[aharon-abramson]

Added the last commit here by mistake.