Add detailed Instructions to Binary Search Tree

[pwadsworth]

Adds instructions to clarify what the exercise expects the student to do by specifying the expected inputs/outputs and intent of each stub function.

[github-actions]

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[glennj]

Not every track that implements this exercises has "functions" (there may different names for it). Not every track will use the same function names.

Track-specific language does not belong in problem-specifications.