Add more docs to WorldInspectorPlugin and update world.rs example

[DanilaMihailov]

Great plugin, good docs! But I still got confused, so I decided to add a little more docs and update example for editing custom components with WorldInspectorPlugin

You can see my confusion in this issue #12

English is not my first language, so please correct my phrasing where needed :)

[jakobhellermann]

Thank you very much! I agree, that part needed more documentation.