Generalize item about edition test annotations

In our authoring guide, we describe how to control the edition for a
test.  This was written as though the only two editions were 2015 and
2018.  Let's update the wording to make this more general.

Author: traviscross

docs/authoring.md

@@ -47,7 +47,7 @@ See <https://rust-lang.github.io/mdBook/format/theme/syntax-highlighting.html#su
 
 Rust examples are tested via rustdoc, and should include the appropriate annotations:
 
-* `edition2015` or `edition2018` --- If it is edition-specific (see `book.toml` for the default).
+* `edition2015`, `edition2018`, etc. --- If it is edition-specific (see `book.toml` for the default).
 * `no_run` --- The example should compile successfully, but should not be executed.
 * `should_panic` --- The example should compile and run, but produce a panic.
 * `compile_fail` --- The example is expected to fail to compile.