Explain how (and why) JSDoc merges adjacent comments

[hegemonic]

We should update the Getting Started page to explain how, and why, we merge adjacent comment blocks. In brief, it's to help users document overloaded methods.

Here's the intended use case:

/**
 * Get the foo.
 *//**
 * Set the foo.
 * @param {string} newFoo - The new foo.
 */
function foo(newFoo) {
    if (newFoo === undefined) {
        return foo;
    }

    foo = newFoo;
}

The docs may cover overloaded methods elsewhere (I haven't checked), but since the Getting Started page talks about JSDoc comment syntax, we should mention this behavior there.