rustdoc book on codeblock hash escaping

Author: lorepozo

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

@@ -170,6 +170,23 @@ By repeating all parts of the example, you can ensure that your example still
 compiles, while only showing the parts that are relevant to that part of your
 explanation.
 
+The `#`-hiding of lines can be prevented by using two consecutive hashes
+`##`. This only needs to be done with with the first `#` which would've
+otherwise caused hiding. If we have a string literal like the following,
+which has a line that starts with a `#`:
+
+```rust
+let s = "foo
+## bar # baz";
+```
+
+We can document it by escaping the initial `#`:
+
+```text
+/// let s = "foo
+/// ## bar # baz";
+```
+
 
 ## Using `?` in doc tests