Add API documentation guidelines in to-contribute

docs/src/to-contribute/style-guidelines/rust-guidelines.md

@@ -1 +1,14 @@
-# Rust Guidlines
+# Rust Guidelines
+
+## API Documentation Guidelines
+
+API documentation describes the meanings and usage of APIs,
+and will be rendered into web pages by rustdoc.
+
+It is necessary to add documentation to all public APIs,
+including crates, modules, structs, traits, functions, macros, and more.
+The use of the `#[warn(missing_docs)]` lint enforces this rule.
+
+Asterinas adheres to the API style guidelines of the Rust community.
+The recommended API documentation style can be found at
+[how-to-write-documentation](https://doc.rust-lang.org/rustdoc/how-to-write-documentation.html).