add note about using doc comments for help messages (texitoi/structopt#1)
//! if not specified:
//!
//! - `name`: The binary name displayed in help messages. Defaults
- to the crate name given by Cargo.
+//! to the crate name given by Cargo.
//! - `version`: Defaults to the crate version given by Cargo.
//! - `author`: Defaults to the crate author name given by Cargo.
//! - `about`: Defaults to the crate description given by Cargo.
//! .default_value("42")
//! ```
//!
+//! ## Help messages
+//!
+//! Help messages for the whole binary or individual arguments can be
+//! specified using the `about` attribute on the struct/field, as we've
+//! already seen. For convenience, they can also be specified using
+//! doc comments. For example:
+//!
+//! ```ignore
+//! #[derive(StructOpt)]
+//! #[structopt(name = "foo")]
+//! /// The help message that will be displayed when passing `--help`.
+//! struct Foo {
+//! ...
+//! #[structopt(short = "b")]
+//! /// The description for the arg that will be displayed when passing `--help`.
+//! bar: String
+//! ...
+//! }
+//! ```
+//!
//! ## Subcomamnds
//!
//! Some applications, like `git`, support "subcommands;" an extra command that
