doc/ref: mention that newlines can be escaped in multiline strings
This behaviour mirrors the behaviour of multiline strings in Swift.
Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Iac15f3d0ba050e9f5d12dd388504eb8723939e27
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/537299
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
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