add some detail re: referencing .assign() and possible memory leaks

[heckj]

Seems like a point of some possible confusion, it's bit at least a few people -

using .assign() captures the reference it's handed with a strong link - not "[weak self]" kind of semantics. Detail at https://forums.swift.org/t/does-assign-to-produce-memory-leaks/29546/9

More that the expectation of how it's handled might be made more clear - it's not really a like so much as a misunderstanding that seems common