fix text describing edition range testing

Author: tshepang

src/doc/rustc-dev-guide/src/tests/directives.md

@@ -264,11 +264,16 @@ Consider writing the test as a proper incremental test instead.
 
 #### The edition directive
 
-The `//@ edition` directive can take an exact edition, a bounded half-open range of editions or a left-bounded half-open range of editions, this affects which edition is used by `./x test` to run the test. For example:
+The `//@ edition` directive can take an exact edition, a bounded range of editions, or a left-bounded half-open range of editions.
+This affects which edition is used by `./x test` to run the test.
+
+For example:
 
 - A test with the `//@ edition: 2018` directive will only run under the 2018 edition.
-- A test with the `//@ edition: 2015..2021` directive can be run under both the 2015 and 2018 editions. However, CI will only run the test with the lowest edition possible (2015 in this case).
-- A test with the `//@ edition: 2018..` directive will run under any edition greater or equal than 2018. However, CI will only run the test with the lowest edition possible (2018 in this case).
+- A test with the `//@ edition: 2015..2021` directive can be run under the 2015, 2018, and 2021 editions.
+  However, CI will only run the test with the lowest edition in the range (which is 2015 in this example).
+- A test with the `//@ edition: 2018..` directive will run under 2018 edition or greater.
+  However, CI will only run the test with the lowest edition in the range (which is 2018 in this example).
 
 You can also force `./x test` to use a specific edition by passing the `-- --edition=` argument. However, tests with the `//@ edition` directive will clamp the value passed to the argument. For example, if we run `./x test -- --edition=2015`: