Apply a suggestion from @aphillips

Author: stasm

spec/formatting.md

@@ -216,6 +216,7 @@ the following steps are taken:
    - If the _option_'s right-hand side successfully resolves to a value,
      bind the _identifier_ of the _option_ to the resolved value in the mapping.
    - Otherwise, bind the _identifier_ of the _option_ to an unresolved value in the mapping.
+     Implementations MAY later remove this value before calling the _function_.
      (Note that an Unresolved Variable error will have been emitted.)
 
 4. Call the function implementation with the following arguments: