docs: Update tests chapter for Termination stabilization

Author: ehuss

src/doc/rustc/src/tests/index.md

@@ -15,7 +15,8 @@ fn it_works() {
 ```
 
 Tests "pass" if they return without an error. They "fail" if they [panic], or
-return a [`Result`] with an error.
+return a type such as [`Result`] that implements the [`Termination`] trait
+with a non-zero value.
 
 By passing the [`--test` option] to `rustc`, the compiler will build the crate
 in a special mode to construct an executable that will run the tests in the
@@ -304,6 +305,7 @@ Experimental support for using custom test harnesses is available on the
 [`libtest`]: ../../test/index.html
 [`main` function]: ../../reference/crates-and-source-files.html#main-functions
 [`Result`]: ../../std/result/index.html
+[`Termination`]: ../../std/process/trait.Termination.html
 [`test` cfg option]: ../../reference/conditional-compilation.html#test
 [attribute-ignore]: ../../reference/attributes/testing.html#the-ignore-attribute
 [attribute-should_panic]: ../../reference/attributes/testing.html#the-should_panic-attribute