add explainatio for cargo

Author: harryhanYuhao

src/doc/rustdoc/src/what-is-rustdoc.md

@@ -34,6 +34,9 @@ the main page is located in `doc/lib/index.html`. If you open that up in
 a web browser, you will see a page with a search bar, and "Crate lib" at the
 top, with no contents.
 
+You can also use `cargo doc` to generate documentation for the whole project. 
+See [Using rustdoc with Cargo](#rustdoc-cargo).
+
 ## Configuring rustdoc
 
 There are two problems with this: first, why does it
@@ -70,16 +73,17 @@ check it out! It should show a link to the `foo` function's page, which
 is located at `doc/docs/fn.foo.html`. On that page, you'll see the "foo is
 a function" we put inside the documentation comment in our crate.
 
-## Using rustdoc with Cargo
+## <a name="rustdoc-cargo"></a> Using rustdoc with Cargo
 
 Cargo also has integration with `rustdoc` to make it easier to generate
 docs. Instead of the `rustdoc` command, we could have done this:
 
 ```bash
-$ cargo doc
+$ cargo doc 
+$ cargo doc --open # automatically open
 ```
 
-Internally, this calls out to `rustdoc` like this:
+Internally, `cargo doc` calls out to `rustdoc` like this:
 
 ```bash
 $ rustdoc --crate-name docs src/lib.rs -o <path>/docs/target/doc -L