Document the use_alloc feature

Author: Philippe-Cholet

src/lib.rs

@@ -37,7 +37,11 @@
 //! - `use_std`
 //!   - Enabled by default.
 //!   - Disable to compile itertools using `#![no_std]`. This disables
-//!     any items that depend on collections (like `chunk_by`, `unique`,
+//!     any item that depend on allocations (see the `use_alloc` feature)
+//!     and hash maps (like `unique`, `counts`, `into_grouping_map` and more).
+//! - `use_alloc`
+//!   - Enabled by default.
+//!   - Enables any item that depend on allocations (like `chunk_by`,
 //!     `kmerge`, `join` and many more).
 //!
 //! ## Rust Version