fix: errors in jsdoc comments, return type of isLeaf()

[pastelmind]

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Description

I found and fixed several incorrect JSDoc comments, such as:

• Incorrect return types (e.g. returns null instead of undefined, or vice versa)
• Function parameters that no longer exist
• Function parameters whose shape has changed
• Incorrect code in @example

I also fixed Tree#isLeaf() so that it always returns a boolean.

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