Some externalDocs missing from OpenAPI document

[lcawl]

The @ext_doc_id modifier was added in #3028
While using it in #3023, it was noticeable that some occurrences were correctly shown in the schema.json (by running make generate) but missing from the OpenAPI documents (by running make transform-to-openapi).

For example, if I add the following in specification/_global/search/_types/FieldCollapse.ts:

/**
 * @ext_doc_id collapse-search-results
 */
export class FieldCollapse {
...

... The schema.json contains:

      "kind": "interface",
      "extDocId": "collapse-search-results",
      "extDocUrl": "https://www.elastic.co/guide/en/elasticsearch/reference/{branch}/collapse-search-results.html",
      "name": {
        "name": "FieldCollapse",
        "namespace": "_global.search._types"
...

But there is no corresponding change to the OpenAPI document.

I did not identify the pattern of why certain occurrences were missing, but it will warrant further investigation if we implement more of those links. It might be as simple as neglecting to add support for @ext_doc_id on one of the specification data types. Alternatively, it seems to me it might reasonably be related to the sibling issue described in #2918

This issue serves as a reminder to dig deeper into this issue so we make full use of all those links we're adding.