doc: list Unsupported Directory Import resolve err

PR-URL: #37032
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: guybedford

doc/api/esm.md

@@ -1012,6 +1012,8 @@ The resolver can throw the following errors:
   subpath in the package for the given module.
 * _Package Import Not Defined_: Package imports do not define the specifier.
 * _Module Not Found_: The package or module requested does not exist.
+* _Unsupported Directory Import_: The resolved path corresponds to a directory,
+  which is not a supported target for module imports.
 
 ### Resolver Algorithm Specification