Rollup merge of #50852 - mandeep:fix-rustdoc-example-testing, r=GuillaumeGomez

Add doc comment to hiding portions of code example

fixes #50816

Not sure if this is all that's needed, but I think it's a good start. One thing to note is that the code block is a text block where it could possibly be a rust block.

Author: Mark-Simulacrum

src/doc/rustdoc/src/documentation-tests.md

@@ -79,15 +79,17 @@ from your example, but are important to make the tests work. Consider
 an example block that looks like this:
 
 ```text
-/// Some documentation.
-# fn foo() {}
+/// /// Some documentation.
+/// # fn foo() {} // this function will be hidden
+/// println!("Hello, World!");
 ```
 
 It will render like this:
 
 ```rust
 /// Some documentation.
 # fn foo() {}
+println!("Hello, World!");
 ```
 
 Yes, that's right: you can add lines that start with `# `, and they will