tree: document sorted nature of random trees

[crisman]

For the use that the tour makes of "golang.org/x/tour/tree" it is important that the generated trees are sorted. While New() does generate sorted trees it is current documented as "returns a new, random binary tree" and many readers will assume "random" implies "non-sorted".

Update doc that trees are "randomly-structured, sorted" to clarify what is random and that trees are sorted.

[gopherbot]

This PR (HEAD: 7c6a425f26e85a52c104ce84411ab3727de830f0) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/tour/+/478315 to see it.

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