Document situation to be signaled

Lame formatters cannot distinguish between when there are syntax errors and
when code should be reformatted.

CUSTOMIZATION.md

@@ -159,6 +159,8 @@ As output, this script should emit whatever the programs it calls emit.
 - 0: file wouldn't be reformatted
 - 1: file would be reformatted
 - 2: file has at least one syntax error
+- 3: file would be reformatted or it has at least one syntax error but we
+  cannot distinguish which
 - 252: Unexpected return value from the tool; that is, the value of `$?` is
   unexpected and thus unhandled
 - 253: Unexpected output from the tool; that is, the output emitted by the
@@ -176,8 +178,8 @@ Even less unsurprisingly, Laravel pint gets it worse than all the others I
 checked; it doesn't distinguish files that need to be reformatted from files
 with actual syntax errors.
 
-Anyway, `gofmt` is the obviously the model to follow.  So we should
-probably drop `1`.)
+If possible, then, we should return 1 or 2 when relevant.  When they can't be
+distinguished, return 3.)
 
 
 ### in `do_format` mode