[concept.regularinvocable] Adding examples for regular_invocable would be helpful

It would be very helpful to having examples on what is allowed in regular_invocable.

Here is a draft:

int g;
int f1(int* p, int& r) { // regular_invocable
    ++g;
    ++*p;
    // r is not modified
    return 0;
}
int f2(int* p, int& r) { // not regular_invocable
    ++r;
    return 0;
}
struct F {
    mutable int m;
    int* p;
    int& r;
    int operator()() const { // regular_invocable
        ++m;
        ++*p;
        ++r;
        return 0;
    }
};

Please note that I do not understand regular_invocable well (so I request to add examples ;) ) and the regular_invocable/non-regular_invocable distinction may be incorrest.

This is related to GB206 (cplusplus/nbballot#204).

Start with the regular_invocable concept wording in the draft and the related GB206 discussion in cplusplus/nbballot#204. Check whether the proposed f1, f2, and F examples correctly illustrate the distinction, then add accurate examples to the relevant draft section; done means the examples clearly match the normative wording.