Auto merge of #29089 - kickinbahk:edit-testing-docs, r=steveklabnik

When going through the docs, it is not clear that binary files cannot be tested. Additionally, it is hard to find the proper structure of a Rust crate and it took me several hours of looking through the docs to find the crates and modules section. I think we can link to it from here and it will be beneficial to those who are coming to the language.

Author: bors

src/doc/trpl/testing.md

@@ -504,3 +504,6 @@ you add more examples.
 
 We haven’t covered all of the details with writing documentation tests. For more,
 please see the [Documentation chapter](documentation.html)
+
+One final note: Tests *cannot* be run on a binary file. To see more on file arrangement see the [Crates and Modules](crates-and-modules.html) section.
+