Clarify that modifyproperty! does not call convert

[tkf]

Is this behavior documented somewhere?

julia> mutable struct Atomic{T}; @atomic x::T; end

julia> a = Atomic(0);

julia> @atomic a.x = 0.0;

julia> @atomic a.x += 0.0;
ERROR: TypeError: in modifyfield!, expected Int64, got a value of type Float64

I don't know if we should fix the documentation or the implementation. Or maybe note it as a possible future fix. So, just as a starter, I implemented the easiest change which is to simply document it.

[tkf]

Sorry, I should have clarified in the OP, but I think @vtjnash needed to approve this (although it's very likely the intended direction).

[aviatesk]

yep sorry for my hasty approval😅

[tkf]

No worries :) I just quickly talked to @vtjnash but it wasn't clear what is the best direction. So, reverting it to play on the safer side: #45209