Update the book to explicitly mention that `impl` functions can't call each other yet.
## Known Issues
-* [#870](https://github.com/FuelLabs/sway/issues/870): All `impl` blocks need to be defined before any of the functions they define can be called.
+* [#870](https://github.com/FuelLabs/sway/issues/870): All `impl` blocks need to be defined before any of the functions they define can be called. This includes sibling functions in the same `impl` declaration, i.e., functions in an `impl` can't call each other yet.
## Missing Features
