`ComposeUnstableReceiver` only for non-Unit returning functions

[blipinsk]

This PR relates to this issue: https://github.com/slackhq/compose-lints/issues/467

A @Composable function can never be skippable if it returns non-Unit. Raising the lint violation in such case is misleading because there's nothing we can do about the receiver type that would make the function skippable.

Additionally, I added a suggestion (we might as well remove it if you don't like it) for docs, to clarify that Strong Skipping doesn't really work well with the rule (link).

[salesforce-cla[bot]]

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