Adjust wording rust-lang#40301

Author: projektir

src/doc/book/src/documentation.md

@@ -170,8 +170,6 @@ more than one section:
 # fn foo() {}
 ```
 
-Let's discuss the details of these code blocks.
-
 #### Code block annotations
 
 To write some Rust code in a comment, use the triple graves:
@@ -183,6 +181,9 @@ To write some Rust code in a comment, use the triple graves:
 # fn foo() {}
 ```
 
+This will add code highlighting. If you are only showing plain text, put `text`
+instead of `rust` after the triple graves (see below).
+
 ## Documentation as tests
 
 Let's discuss our sample example documentation: