Document how self: Self receiver types *actually* interact with object safety

[steffahn]

Fixes #1247

The description here on how this works is based purely on experiments on what the compiler does or does not accept (without unstable features), not on any inspection of the implementation in rustc. The term of “implicitly non-dispatchable functions” is one I just made up, as far as I’m aware, because it makes sense in this context. The motivation of future-compatibility is just my guess, too.

I might want to read up some actual sources around relevant RFCs and/or PRs, for precise (intended) behavior, terminology, and motivation of the behavior of self: Self but not Self: Sized methods in traits.

For review, especially if any of my “guesses” here are wrong, I’d thus also appreciate relevant links to relevant discussions to learn from.

[steffahn]

cc @QuineDot since you recently opened #1445, so you might have some ideas or feedback here