Add explaination how hollow works

[OneLiteFeather]

For which plugin is this issue?

• FastAsyncWorldEdit (2.6.2)

What should be added in documentation:

There is no good explaination how //hollow works as stated here: IntellectualSites/FastAsyncWorldEdit#2187
It needs a secondary command to work for the "shell" the user wants to create.

Why should it be added:

This should be added to prevent confusion and to help people in this case.