docs: document external dependencies for sys crate

Author: scootermon

openthread-sys/src/lib.rs

@@ -1,3 +1,26 @@
+//! Low-level bindings to the OpenThread library.
+//!
+//! # External dependencies
+//!
+//! Openthread depends on functions from the C standard library. These are not
+//! provided directly by this crate, but are expected to be linked by the user
+//! in some way.
+//!
+//! The following functions are required:
+//!
+//! - `int iscntrl(int c)`
+//! - `int isprint(int c)`
+//! - `int isupper(int c)`
+//! - `[[noreturn]] void exit(int status)`
+//!
+//! The following functions are required, but this crate already provides an
+//! implementation for them:
+//!
+//! - `int snprintf(char* s, size_t n, const char* format, ...)`
+//! - `int vsnprintf(char* s, size_t n, const char* format, va_list arg)`
+//!
+//! Note that this list is likely to change over time.
+
 #![no_std]
 #![allow(unknown_lints)]