Use `fail` in `Error` example
Fix error in function spec
Reword descriptions of `return/1` and `bind/2`
Tweak documenation of `Monad.Error` module
Fix typo
Tweak documentation of `Monad.Maybe` module
Fix wrong module name in documentation
Fix spello
Tweak `Monad.Reader` documentation
Tweak `Monad.State` documentation