clarify how whichever target is found first will take priority.

src/doc/rustc/src/targets/custom.md

@@ -22,7 +22,9 @@ When `rustc` is given an option `--target=TARGET` (where `TARGET` is any string)
 1. if `TARGET` is the name of a built-in target, use that
 2. if `TARGET` is a path to a file, read that file as a json target
 3. otherwise, search the colon-seperated list of directories found
-   in the `RUST_TARGET_PATH` environment variable for a file named
-   `TARGET.json`.
+   in the `RUST_TARGET_PATH` environment variable from left to right
+   for a file named `TARGET.json`.
 
+These steps are tried in order, so if there are multple potentially valid
+interpretations for a target, whichever is found first will take priority.
 If none of these methods find a target, an error is thrown.