document tasks module

xtask/src/main.rs

@@ -1,5 +1,12 @@
-//! FIXME: write short doc here
+//! See https://github.com/matklad/cargo-xtask/.
-
+//!
+//! This binary defines various auxiliary build commands, which are not
+//! expressible with just `cargo`. Notably, it provides `cargo xtask codegen`
+//! for code genetaiont and `cargo xtask install` for installation of
+//! rust-analyzer server and client.
+//!
+//! This binary is integrated into the `cargo` command line by using an alias in
+//! `.cargo/config`.
 mod help;
 
 use core::fmt::Write;