Describe filename ordering more accurately

src/syntax_mapping/builtins/README.md

@@ -95,11 +95,11 @@ like this:
 
 ## Ordering
 
-At compile time, all TOML files are processed in filesystem order. So
-`00-foo.toml` takes precedence over `10-bar.toml`, which takes precedence over
-`20-baz.toml`, and so on. This may be occasionally useful for creating high/low
-priority rules, such as in the aforementioned example of explicitly mapping
-`*.conf` files to unknown.
+At compile time, all TOML files are processed in lexicographical filename order.
+So `00-foo.toml` takes precedence over `10-bar.toml`, which takes precedence
+over `20-baz.toml`, and so on. This may be occasionally useful for creating
+high/low priority rules, such as in the aforementioned example of explicitly
+mapping `*.conf` files to unknown.
 
 Generally this should not be much of a concern, since rules should be written as
 specifically as possible for each application.