docs: document the required C functions for the -sys crate

mbedtls-rs-sys/src/lib.rs

@@ -1,7 +1,61 @@
-//! Raw bindings to the MbedTLS library
+//! Raw bindings to the MbedTLS library.
+//!
+//! # External dependencies
+//!
+//! MbedTLS depends on the following functions from the C standard library:
+//!
+//! - `calloc`
+//! - `free`
+//! - `memchr`
+//! - `memcmp`
+//! - `memcpy`
+//! - `memmove`
+//! - `memset`
+//! - `printf`
+//! - `putchar`
+//! - `puts`
+//! - `rand`
+//! - `snprintf`
+//! - `strchr`
+//! - `strcmp`
+//! - `strlen`
+//! - `strncmp`
+//! - `strncpy`
+//! - `strstr`
+//! - `vsnprintf`
+//!
+//! This crate does not provide implementations of these functions. It's up to
+//! the consumer to ensure that they are available at link time. On most
+//! platforms, these functions are provided by the host environment's libc
+//! implementation, so no additional work is needed.
+//! On embedded platforms, you may need to link a suitable libc implementation
+//! like [tinyrlibc](https://github.com/rust-embedded-community/tinyrlibc).
+//!
+//! Note that depending on what features of MbedTLS you use, you won't need
+//! all of these functions. The list represents the full set of functions that
+//! MbedTLS may call.
+//! The linker error messages will indicate which specific functions are
+//! missing, so you can focus on providing those first.
+//!
+//! Additionally, the following list of MbedTLS functions are left
+//! unimplemented in this crate:
+//!
+//! - `mbedtls_platform_zeroize`
+//! - `mbedtls_psa_external_get_random`
+//!
+//! These functions are provided by `mbedtls-rs`. If you're building C code
+//! that depends on MbedTLS, make sure to link against `mbedtls-rs` to get
+//! these implementations.
 
 #![no_std]
 #![allow(clippy::uninlined_format_args)]
+#![allow(
+    rustdoc::bare_urls,
+    rustdoc::broken_intra_doc_links,
+    rustdoc::invalid_html_tags,
+    rustdoc::invalid_rust_codeblocks,
+    reason = "Documentation is generated from the C header files"
+)]
 #![allow(unknown_lints)]
 
 pub use bindings::*;