Docs: Document feature flags #2425
Description
There's no standard place to document feature flags, however there's other libraries that will list their feature flags in their crate docs.
For instance tokio in lib.rs
//! # Feature flags
//!
//! Tokio uses a set of [feature flags] to reduce the amount of compiled code. It
//! is possible to just enable certain features over others. By default, Tokio
//! does not enable any features but allows one to enable a subset for their use
//! case. Below is a list of the available feature flags. You may also notice
//! above each function, struct and trait there is listed one or more feature flags
//! that are required for that item to be used. If you are new to Tokio it is
//! recommended that you use the `full` feature flag which will enable all public APIs.
//! Beware though that this will pull in many extra dependencies that you may not
//! need.
Another example is rustls in lib.rs
//! # Crate features
//! Here's a list of what features are exposed by the rustls crate and what
//! they mean.
//!
//! - `std` (enabled by default): enable the high-level (buffered) Connection API and other functionality
//! which relies on the `std` library.
As quinn has 20 different feature flags (only 4 less than tokio!) it would be very helpful if the different feature flags were documented in the create docs.