Add explanation about persistent collection to doc

[ayyubid]

I'm not familiar with functional programming and just a Kotlin programmer.
It would be awesome if someone could add a simple explanation about what a Persistent Collection is and how it work to doc. Thanks!

[mgroth0]

I feel the same. I would also like to know whether persistent collections are are memory safe. I understand that when you use certain operations you may get a result that shares storage with the input. But can we still feel safe that the input and all its elements can be garbage collected normally? Or does the output hold references to elements in the input, even if those elements aren't in the output?