Add section about "Example Output"

Author: blyxyas

CHANGELOG.md

@@ -4552,6 +4552,7 @@ Released 2018-09-13
 [`fn_to_numeric_cast`]: https://rust-lang.github.io/rust-clippy/master/index.html#fn_to_numeric_cast
 [`fn_to_numeric_cast_any`]: https://rust-lang.github.io/rust-clippy/master/index.html#fn_to_numeric_cast_any
 [`fn_to_numeric_cast_with_truncation`]: https://rust-lang.github.io/rust-clippy/master/index.html#fn_to_numeric_cast_with_truncation
+[`foo_functions`]: https://rust-lang.github.io/rust-clippy/master/index.html#foo_functions
 [`for_kv_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#for_kv_map
 [`for_loop_over_option`]: https://rust-lang.github.io/rust-clippy/master/index.html#for_loop_over_option
 [`for_loop_over_result`]: https://rust-lang.github.io/rust-clippy/master/index.html#for_loop_over_result

book/src/development/writing_tests.md

@@ -86,6 +86,40 @@ As we gradually implement our lint logic, we will keep running this UI test comm
 Clippy will begin outputting information that allows us to check if the output is
 turning into what we want it to be.
 
+### Example output
+
+As our `foo_functions` lint is tested, the output would look something like this:
+
+```
+failures:
+
+---- compile_test stdout ----
+normalized stderr:
+error: function called "foo"
+  --> $DIR/foo_functions.rs:6:12
+   |
+LL |     pub fn foo(&self) {}
+   |            ^^^
+   |
+   = note: `-D clippy::foo-functions` implied by `-D warnings`
+
+error: function called "foo"
+  --> $DIR/foo_functions.rs:13:8
+   |
+LL |     fn foo(&self) {}
+   |        ^^^
+
+error: function called "foo"
+  --> $DIR/foo_functions.rs:19:4
+   |
+LL | fn foo() {}
+   |    ^^^
+
+error: aborting due to 3 previous errors
+```
+
+Note the *failures* label at the top of the fragment, we'll get rid of it (saving this output) in the next section.
+
 > _Note:_ You can run multiple test files by specifying a comma separated list:
 > `TESTNAME=foo_functions,bar_methods,baz_structs`.
 ### `cargo dev bless`