Improve docs on the const fn API

I randomly stumbled upon it and realised that it's not that clear.

Author: meithecatte

src/lib.rs

@@ -62,9 +62,11 @@
 //! over `T: BitFlag`, you will be met with an error explaining that currently,
 //! the only allowed trait bound for a `const fn` is `?Sized`. You will probably
 //! want to write a separate implementation for `BitFlags<T, u8>`,
-//! `BitFlags<T, u16>`, etc — probably generated by a macro.
-//! This strategy is often used by `enumflags2` itself; to avoid clutter, only
-//! one of the copies is shown in the documentation.
+//! `BitFlags<T, u16>`, etc — best accomplished by a simple macro.
+//!
+//! **Documentation considerations:** The strategy described above is often used
+//! by `enumflags2` itself. To avoid clutter in the auto-generated documentation,
+//! the implementations for widths other than `u8` are marked with `#[doc(hidden)]`.
 //!
 //! ## Customizing `Default`
 //!