Docs for for_each() should be clearer about forbidding "any non-constant operation"

[Tony Lewis <tonyelewis@…>]

The docs for Range's (single-range) for_each() function specify the requirement:

UnaryFunction does not apply any non-constant operation through its argument.

I think this wording could it be clearer. Does it mean that the it mustn't modify the value passed to it? If not, I think it would benefit from rewording. If so, isn't that quite different to std::for_each(). If that's deliberate, I suggest it's worth a brief note highlighting and justifying the difference. Otherwise, I suggest it's worth fixing.

Thanks very much.