document local methods, warn about branches

[tpapp]

Briefly explain local methods and limitations in the manual.

The main motivation is people running into variants of #15602, this should be documented.

[matthias314]

Maybe one could write "leads to undefined behavior" instead of "prevents various compiler optimizations" because such code can actually produce errors. Also, it might be worthwhile to mention that "conditional definitions" include those following a return statement, as in

function c2()
    function f end
    return f
    f() = 0
end

c2()  # UndefVarError: f not defined

[tpapp]

@matthias314: thanks, I implemented this. I am not totally sure whether to document the ideal we are going for (once #15602 is fixed, ideally all of these things should error), or the status quo (which is indeed undefined behavior or an error message that is hard to interpret).

We can always edit this section in the future though, but it would be important to mention this in the manual in some form.

[tpapp]

Friendly bump, I wonder if someone could review this short docs PR.

[KristofferC]

I think describing this as generally "undefined behavior "is a bit harsh. It is typically used to mean absolutely anything can happen and your system can be in an arbitrary state. I wonder if this could be softened somewhat.

[tpapp]

What about "unexpected errors"?