It took me a little while to figure out how to use the `SystemParam` derive macro to easily create my own params. So I figured I'd add some docs and an example with what I learned.
- Fixed a bug in the `SystemParam` derive macro where it didn't detect the correct crate name when used in an example (no longer relevant, replaced by #1426 - see further)
- Added some doc comments and a short example code block in the docs for the `SystemParam` trait
- Added a more complete example with explanatory comments in examples
Before, when deriving `SystemLabel` for a type without `Clone`, the error message was:
```
the trait `SystemLabel` is not implemented for `&TransformSystem`
```
Now it is
```
the trait `Clone` is not implemented for `TransformSystem`
```
which directly shows what's needed to fix the problem.
