Update document for Creating New Generators section (JuliaLang#33070)

document what copy should do for RNG's, fixes JuliaLang#33008

stdlib/Random/docs/src/index.md

@@ -285,7 +285,10 @@ The API is not clearly defined yet, but as a rule of thumb:
  should be defined for this specific RNG, if they are needed;
 2) other documented `rand` methods accepting an `AbstractRNG` should work out of the box,
  (provided the methods from 1) what are relied on are implemented),
- but can of course be specialized for this RNG if there is room for optimization.
+ but can of course be specialized for this RNG if there is room for optimization;
+3) `copy` for pseudo-RNGs should return an independent copy that generates the exact same random sequence as the
+ original from that point when called in the same way. When this is not feasible (e.g. hardware-based RNGs),
+ `copy` must not be implemented.
 
 Concerning 1), a `rand` method may happen to work automatically, but it's not officially
 supported and may break without warnings in a subsequent release.