auto merge of #15374 : steveklabnik/rust/comments, r=brson

I'm leaving off `rustdoc` usage because it won't work unless this is a `pub fn`, and I want to talk about public/private in the context of modules. I'm also not mentioning `//!` because it is exclusively used to provide the overview of a module.

src/doc/guide.md

@@ -940,11 +940,54 @@ fn foo(x: int) -> int {
 There are some additional ways to define functions, but they involve features
 that we haven't learned about yet, so let's just leave it at that for now.
 
+
 ## Comments
 
-return
+Now that we have some functions, it's a good idea to learn about comments.
+Comments are notes that you leave to other programmers to help explain things
+about your code. The compiler mostly ignores them.
 
-comments
+Rust has two kinds of comments that you should care about: **line comment**s
+and **doc comment**s.
+
+```{rust}
+// Line comments are anything after '//' and extend to the end of the line.
+
+let x = 5i; // this is also a line comment.
+
+// If you have a long explanation for something, you can put line comments next
+// to each other. Put a space between the // and your comment so that it's
+// more readable.
+```
+
+The other kind of comment is a doc comment. Doc comments use `///` instead of
+`//`, and support Markdown notation inside:
+
+```{rust}
+/// `hello` is a function that prints a greeting that is personalized based on
+/// the name given.
+///
+/// # Arguments
+///
+/// * `name` - The name of the person you'd like to greet.
+///
+/// # Example
+///
+/// ```rust
+/// let name = "Steve";
+/// hello(name); // prints "Hello, Steve!"
+/// ```
+fn hello(name: &str) {
+    println!("Hello, {}!", name);
+}
+```
+
+When writing doc comments, adding sections for any arguments, return values,
+and providing some examples of usage is very, very helpful.
+
+You can use the `rustdoc` tool to generate HTML documentation from these doc
+comments. We will talk more about `rustdoc` when we get to modules, as
+generally, you want to export documentation for a full module.
 
 ## Compound Data Types