Validate runtime fields sans scripts

[lockewritesdocs]

Background:
#84184 introduced runtime fields in the index template. Users can define a runtime field with or without a script. #85204 adds text to explain the behavior of a runtime field without a script:

Runtime fields without a script retrieve values from a field with the same name in _source. If a field with the same name doesn’t exist, no values are returned when a search request includes the runtime field.

Describe the feature:
If a user defines a runtime field and no field with the same name exists in _source, we should display an informational message to inform them. Something like:

No field with this name exists in _source. Search requests including this runtime field will not return any values.

Describe a specific use case for the feature:
A user defines a runtime field in the index template without a script. They later run a query that includes this runtime field, but no values are returned. They are curious why no values were returned.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[lockewritesdocs]

I talked to @javanna about this validation and think that we should not do it. Kibana would need to validate both the mapping and _source fields to check whether a field with the same name as the runtime field exists. We can check whether the field exists in the mappings, but because not all _source fields are mapped, we can’t go and check every single document if there is one value for the field. We could, but it would probably be expensive and really slow Kibana’s response time.

Closing this issue and relying on the text introduced in #85204 to guide users on creating runtime fields without scripts.