Mention that to_lowercase isn't perfect

src/ch12-05-working-with-environment-variables.md

@@ -60,7 +60,10 @@ function to lowercase the query and the line before comparing them</span>
 First, we lowercase the `query` string and store it in a shadowed variable with
 the same name. Calling `to_lowercase` on the query is necessary so no matter
 whether the user’s query is `"rust"`, `"RUST"`, `"Rust"`, or `"rUsT"`, we’ll
-treat the query as if it were `"rust"` and be insensitive to the case.
+treat the query as if it were `"rust"` and be insensitive to the case. While
+`to_lowercase` will handle basic Unicode, it won't be 100% accurate. If we were
+writing a real application, we'd want to do a bit more work here, but this section
+is about environment variables, not Unicode, so we'll leave it at that here.
 
 Note that `query` is now a `String` rather than a string slice, because calling
 `to_lowercase` creates new data rather than referencing existing data. Say the