docs: Add indicators for methods that are std-only (#53)

* docs: Add std-only indications

* Module-level doc

* fmt

Cargo.toml

@@ -30,3 +30,7 @@ getrandom = { version = "0.2", features = ["js"] }
 rand = "0.8"
 wyhash = "0.5"
 getrandom = "0.2"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]

src/lib.rs

@@ -72,6 +72,7 @@
 //!   the [`with_seed`](Rng::with_seed) method.
 
 #![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 #![forbid(unsafe_code)]
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
 
@@ -79,6 +80,7 @@ use core::convert::{TryFrom, TryInto};
 use core::ops::{Bound, RangeBounds};
 
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 mod global_rng;
 
 #[cfg(feature = "std")]