Fixes smithy-lang#2400.

When a member targets a structure, it becomes a schema reference when
converted to JSON Schema. Previously, we didn't add member docs to the
converted object, possibly because earlier versions of open api or JSON
Schema did not support it. Reading through [this issue](OAI/OpenAPI-Specification#1514)
the [OAI spec](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#reference-object),
and the [JSON Schema Spec](https://json-schema.org/draft/2020-12/json-schema-core#section-8.2.3),
it seems that OpenAPI 3.1 and JSON Schema 2020-12 support the
`description` property alongside `$ref`. I wasn't able to find anything
about whether it is supported in [JSON Schema 07](https://json-schema.org/draft-07/json-schema-release-notes).
This commit adds a new config option, `addReferenceDescriptions` that
will add the `description` property alongside `$ref` when the member has
documentation. I made it opt-in through the config option so we don't
cause any existing documentation to be changed unexpectedly.