Update wasm-rt-impl.h

[whirlicote]

trafficstars

It appears to me that the wasm_rt_impl_try macro should have been defined in the wasm-rt-exceptions.h header.

This pull request tries to improve the documentation on the current situation in order to improve the user experience.

[sbc100]

@SoniEx2

[SoniEx2]

uh, actually we believe this is a bug! think this got overlooked during the implementation of EHv3 (cc @keithw )

further, we believe this code technically triggers UB, ref.: https://www.iso-9899.info/n1570.html#7.13.1.1