Rollup merge of rust-lang#28761 - steveklabnik:gh28082, r=brson

We don't completely cover documentation tests in the testing chapter,
since we cover them in the documentation chapter. So make sure people
know that.

Fixes rust-lang#28082

src/doc/trpl/testing.md

@@ -502,3 +502,5 @@ documentation tests: the `_0` is generated for the module test, and `add_two_0`
 for the function test. These will auto increment with names like `add_two_1` as
 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)