A section on the implications of including worlds

In https://github.com/bytecodealliance/component-docs/issues/245 a user encountered a bump when trying to import functionality from `wasi:http` that I'd guess will occur again.

Not sure if this belongs in the subsection on worlds, since it could also be part of https://github.com/bytecodealliance/component-docs/issues/240, or there could perhaps be a section that pertains to all guest languages, as it probably isn't unique to the rust experience.

The user included `wasi:http/proxy` and didn't realize that they would have to implement the interfaces exported by it, as they were really only after importing the functionality that that world provided, and had no intention of authoring a component that exported any of it's functionality. Ideally we would clearly inform users to reserve `include` usage for when they want to define a world that is a superset of another.

閱讀連結的 issue #245 和 #240，以及現有的 worlds 子章節，以了解圍繞包含 wasi:http/proxy 所回報的困惑。完成的標準是文件清楚地說明 include 用於定義一個超集 world，並區分匯入功能與實作匯出的介面。

由索引模型根據 Issue 內容生成。