Clarify the binary search tree exercise

[haranishi]

The smart pointers exercise implements a binary search tree, but the exercise title, sidebar, and type documentation call it a binary tree. Clarify the distinction and explain the ordering invariant that insert and has rely on. Also describe a binary tree as having at most two children per node.

Fixes #2732.

Validation:

• cargo test --locked -p smart-pointers (3 tests passed).
• mdbook test --chapter smart-pointers/exercise.md and mdbook test --chapter smart-pointers/solution.md.
• mdbook build, including the local link check.
• mdbook serve: the exercise page returns HTTP 200 with the updated title and explanation.
• dprint check on the three changed files and git diff --check.

Local setup: cargo xtask install-tools hit a thiserror_impl E0463 error in Bazel on macOS. The repository tool versions were installed with Cargo for the checks above.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.