Add documentation of fsm function

hazardflow-designs/src/std/interface.rs

@@ -26,6 +26,16 @@ pub trait Interface: Sized {
     /// In particular, for a hazard interface [`I<H, D>`], you must follow the specification described in the "Safety"
     /// section of [`I::fsm`].
     ///
+    /// # How it is compiled?
+    ///
+    /// In the HazardFlow compiler, the fsm function is not actually executed (which would lead to a panic).
+    ///
+    /// Instead, the HazardFlow compiler captures the [High-level IR](https://rustc-dev-guide.rust-lang.org/hir.html)
+    /// generated by the Rust compiler and extracts information about the ingress/egress/state types (`Self`, `E`, `S`)
+    /// and the arguments (`init_state`, `f`) of the fsm function.
+    ///
+    /// Using this information, the HazardFlow compiler generates the corresponding Verilog code.
+    ///
     /// # Type parameters
     ///
     /// - `Self`: The ingress interface type.