Miscellaneous feedback regarding the merge stage spec text

[glen-84]

1. "Let description be the description of argumentA or undefined if none exists."

   Should we use null here instead of undefined?

2. https://graphql.github.io/composite-schemas-spec/draft/#sec-Merge-Argument.Service-A – should this heading be removed?

3. https://github.com/graphql/composite-schemas-spec/blame/main/spec/Section%204%20--%20Composition.md#L3566-L3568

   When only one enum definition is present (after removing any inaccessible ones)

   Referring to the text in parentheses, if one of the definitions was inaccessible, then wouldn't they all be inaccessible, and there would be none left to merge? (for the composite schema, not the execution schema)