Improve reasoning and examples for F.48

[eisenwave]

With guaranteed copy elision, it is now almost always a pessimization to expressly use std::move in a return statement.

The reasoning in F.48 is misleading because it only refers to guaranteed copy elision, but the example listed underneath shows NRVO, for which this elision is not guaranteed.

I think it the reasoning is much better if it is made clear to the reader that a return statement will always move local variables. It is also helpful to distinguish between RVO and NRVO in the example, as the two have optimization differences.