message: Add docs on how cargo-credential finds the action argument.
diff: //! cargo_credential::main(MyCredential);
  //! }
  //! ```
  +//!
  +//! This will determine the action to perform (get/store/erase) by looking at
  +//! the CLI arguments for the first argument that does not start with `-`. It
  +//! will then call the corresponding method of the trait to perform the
  +//! requested action.
  pub type Error = Box<dyn std::error::Error>;
  