[docs] Properties: Explain capturing instance values

Comment on existing text. This: https://github.com/Polymer/lit-element/pull/489/files#diff-f37c02b7dd321fd8fca26553760681d7R16
Is a little vague. I'm not sure a reader unfamiliar with upgrades and this issue already will know what that's talking about. Maybe file an issue to clean up?

@justinfagnani FYI

[nicolejadeyee]

PR was merged and docs have been re-written. closing issue.