Improve the doc

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cvt"
-version = "0.1.0"
+version = "0.1.1"
 authors = ["Marcin Mielniczuk <[email protected]>"]
 edition = "2018"
 license = "Apache-2.0"

README.md

@@ -7,6 +7,12 @@
 [crates-io]: https://crates.io/crates/cvt
 
 This package exposes the `cvt` function used extensively by `libstd` to
-convert platform-specific syscall error codes.
+convert platform-specific syscall error codes to `std::io::Result`.
 
-The code was mostly copied over from Rust libstd.
+Usually syscalls use return values for errors, the conventions differ. For instance,
+on Unix `0` means success on Unix but failure on Windows.
+
+While those conventions are not always followed, they usually are and
+`cvt` is there to reduce the mental bookkeeping and make it easier to handle syscall errors.
+
+The code was mostly copied over from Rust libstd, because the function is not public.

src/lib.rs

@@ -1,3 +1,13 @@
+//! This package exposes the `cvt` function used extensively by `libstd` to
+//! convert platform-specific syscall error codes to `std::io::Result`.
+//!
+//! Usually syscalls use return values for errors, the conventions differ. For instance,
+//! on Unix `0` usually means success on Unix but failure on Windows.
+//! While those conventions are not always followed, they usually are and
+//! `cvt` is there to reduce the mental bookkeeping and make it easier to handle syscall errors.
+//!
+//! The code was mostly copied over from Rust libstd, because the function is not public.
+
 cfg_if::cfg_if! {
     if #[cfg(target_os = "vxworks")] {
         mod vxworks;