[DOCS] EQL/SQL: Document runtime_fields parameter

[jrodewig]

Changes:

• Adds the runtime_fields parameter to the EQL and SQL search API reference
• Adds a runtime fields example for EQL and SQL
• Makes some minor revisions to the runtime_fields parameter definition

Relates to #71356

Previews

• EQL search API ref: https://elasticsearch_71487.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/eql-search-api.html
• EQL runtime fields example: https://elasticsearch_71487.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/eql.html#eql-use-runtime-fields
• SQL parameters ref: https://elasticsearch_71487.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/sql-rest-fields.html
• SQL runtime fields example: https://elasticsearch_71487.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/sql-runtime-fields.html
• runtime_fields parameter in search API ref: https://elasticsearch_71487.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/search-search.html#search-api-body-runtime

[elasticmachine]

Pinging @elastic/es-ql (Team:QL)

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[lockewritesdocs]

👍 to these changes. I like the more succinct description for runtime_mappings.

[astefan]

LGTM with one comment, though.
Thanks.