docs: Update documentation of shiftRight, fixes #537#539

This clarifies the precondition for shiftRight, i.e. a non-zero, non-max to avoid all exceptional/ISA-defined path. shiftRight was initially introduced to optimized GCD/modular inversion in https://github.com/mratsim/constantine/commit/bde619155b34182552a236652842e9be12b863e5. Today it is used in a couple more places and there are no "zero" input expected in constant-time path. In fact all inputs are statically known, the proc doesn't use a static k to avoid code bloat / monomorphization. A version that can handles zero and sizes over the word bitwidth is available as `shiftRight_vartime`.