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MINOR: [C++][Docs] Fix MapBuilder docstring (#39755)

The [current `MapBuilder` documentation](https://arrow.apache.org/docs/cpp/api/builder.html#_CPPv4N5arrow10MapBuilderE) says:
> To use this class, you must append values to the key and item array builders and use the Append function to delimit each distinct map (once the keys and items have been appended)

This contradicts the [docstring for `Append`](https://arrow.apache.org/docs/cpp/api/builder.html#_CPPv4N5arrow10MapBuilder6AppendEv):
> This function should be called before beginning to append elements to the key and item builders

The `Append` documentation is correct; it should be called *before* keys and items have been appended. If `Append` is called after, as the `MapBuilder` docstring suggests, `Finish` results in an empty `Array`.

### What changes are included in this PR?
Documentation only change.

### Are these changes tested?
There are no behavior changes.

### Are there any user-facing changes?
No

Authored-by: Lyndon Shi <9373058+lynshi@users.noreply.github.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>