Improve doc about positional arguments
fix
//! The type of the field gives the kind of argument:
//!
//! Type | Effect | Added method call to `clap::Arg`
-//! ---------------------|--------------------------------------|--------------------------------------
-//! `bool` | `true` if present | `.takes_value(false).multiple(false)`
-//! `u64` | number of times the argument is used | `.takes_value(false).multiple(true)`
-//! `Option<T: FromStr>` | optional argument | `.takes_value(true).multiple(false)`
-//! `Vec<T: FromStr>` | list of arguments | `.takes_value(true).multiple(true)`
-//! `T: FromStr` | required argument | `.takes_value(true).multiple(false).required(!has_default)`
+//! ---------------------|---------------------------------------------------|--------------------------------------
+//! `bool` | `true` if the flag is present | `.takes_value(false).multiple(false)`
+//! `u64` | number of times the flag is used | `.takes_value(false).multiple(true)`
+//! `Option<T: FromStr>` | optional positional argument or option | `.takes_value(true).multiple(false)`
+//! `Vec<T: FromStr>` | list of options or the other positional arguments | `.takes_value(true).multiple(true)`
+//! `T: FromStr` | required option or positional argument | `.takes_value(true).multiple(false).required(!has_default)`
//!
//! The `FromStr` trait is used to convert the argument to the given
//! type, and the `Arg::validator` method is set to a method using
