Document the web feature in lib.rs

src/lib.rs

@@ -38,6 +38,18 @@
 //! [`Rc`]: https://doc.rust-lang.org/std/rc/struct.Rc.html
 //! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
 //! [`PrivateKeyDer::clone_key()`]: https://docs.rs/rustls-pki-types/latest/rustls_pki_types/enum.PrivateKeyDer.html#method.clone_key
+//!
+//! ## Target `wasm32-unknown-unknown` with the `web` feature
+//!
+//! [`std::time::SystemTime`](https://doc.rust-lang.org/std/time/struct.SystemTime.html)
+//! is unavailable in `wasm32-unknown-unknown` targets, so calls to
+//! [`UnixTime::now()`](https://docs.rs/rustls-pki-types/latest/rustls_pki_types/struct.UnixTime.html#method.now),
+//! otherwise enabled by the [`std`](https://docs.rs/crate/rustls-pki-types/latest/features#std) feature,
+//! require building instead with the [`web`](https://docs.rs/crate/rustls-pki-types/latest/features#web)
+//! feature. It gets time by calling [`Performance.now()`](https://developer.mozilla.org/en-US/docs/Web/API/Performance/now).
+//! in the browser. According to the underlying [`web-time`](https://github.com/daxpedda/web-time)
+//! dependency on which the feature is built, "Emscripten is not supported. WASI doesn't require
+//! support as it has it's own native API to deal with std::time."
 
 #![cfg_attr(not(feature = "std"), no_std)]
 #![warn(unreachable_pub, clippy::use_self)]