doc: Explain 'if-exists' options

[fraya]

It's difficult to know what is the difference between #"overwrite", #"new-version" in if-exists and the combination with direction:.

https://github.com/dylan-lang/opendylan/blob/a95679e64414372d2b542a5439bb4934e5d74c55/documentation/source/library-reference/io/streams.rst?plain=1#L1233