Add explaination of what `a valid system configuration` is

## How can Bevy's documentation be improved?

As shown in the discussion at https://github.com/bevyengine/bevy/discussions/21156, the following error occurred when implementing the `Combine` trait:

``error[E0277]: `X` does not describe a valid system configuration``

While that discussion has been resolved, I still do not understand what a valid system configuration is.

The error message alone does not clarify what actions need to be taken. Could you consider explaining what "a valid system configuration" is, or adding a link to that explanation in the error message?