Clarify option resolution regarding fallback option values (#1054)

* Make option resolution return something if `rv` is a fallback value

Fixes #1053.

* Add note and rephrase slightly

Added a note explaining the fallback resolved values are not valid options and the option is thus omitted. I also reworded the emit-bad-option step to make it not look like an if statement missing an else

* Update spec/formatting.md

Co-authored-by: Eemeli Aro <[email protected]>

* Update spec/formatting.md

Co-authored-by: Eemeli Aro <[email protected]>

---------

Co-authored-by: Eemeli Aro <[email protected]>

Author: aphillips

spec/formatting.md

@@ -395,13 +395,17 @@ For each _option_:
    1. Let `id` be the string value of the _identifier_ of the _option_.
    1. Let `rv` be the _resolved value_ of the _option value_.
    1. If `rv` is a _fallback value_:
-      1. If supported, emit a _Bad Option_ error.
+      1. Emit a _Bad Option_ error, if supported.
    1. Else:
       1. If the _option value_ consists of a _literal_:
          1. Mark `rv` as a _literal_ _option value_.
       1. Set `res[id]` to be `rv`.
 1. Return `res`.
 
+> [!NOTE]
+> If the _resolved value_ of an _option value_ is a _fallback value_,
+> the _option_ is intentionally omitted from the mapping of resolved options.
+
 The result of _option resolution_ MUST be a (possibly empty) mapping
 of string identifiers to values;
 that is, errors MAY be emitted, but such errors MUST NOT be fatal.