Cross-reference doc chapter from testing chapter

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)