[over.oper.general] Remove paragraph 8 and make [over.xxx] more self-contained

For a moment, I was wondering whether operator() can be a free function, and it's not as obvious as it could be because of how inelegant the ban is:

• operator() is an operator function by definition.
• It has to be a function call operator function (or other special kind) because of [oper.over.general] p8.
• A function all operator function is an operator() that is also a member function.
• By exhaustively looking through all possible kinds of operator functions and finding that a free operator() is none of them, we can conclude that it cannot be a free function.

This is very convoluted compared to [over.ass] p1:

A simple assignment operator function is a binary operator function named operator=.
A simple assignment operator function shall be a non-static member function.

I think we should apply the same wording strategy for all others mentioned in [oper.over.general] p8. I.e. we apply changes such as:

A function call operator function is a function named operator()
-that is a member function with
+. A function call operator function shall be a member function.
+It may have
an arbitrary number of parameters.
It may have default arguments.

Or in short, instead of having a narrow definition and saying that stuff has to fall into that narrow definition, we have a broader definition and mandate stuff.

This is simpler and has no normative impact.

Read [over.oper.general] p8 and [over.ass] p1, then inspect the operator-function clauses referenced there. Update the relevant wording so each named kind is self-contained, and verify the draft has no normative change.