Fix example which wrongly implied that the upper bound of edition ranges was inclusive

And explicitly state the fact that the upper bound is exclusive since only giving an example is too implicit.

Author: fmease

src/tests/directives.md

@@ -273,7 +273,9 @@ 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 the 2015, 2018, and 2021 editions.
+- A test with the `//@ edition: 2015..2021` directive can be run under the 2015 and the 2018 edition,
+  so the upper bound is exclusive just like in Rust
+  (note that there's no equivalent to Rust's `..=` where the upper bound is inclusive).
   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).