docs: inputs: several plugin docs need updating with HTTP listener workers details

[eschabell]

pipeline/inputs/http.md

• Add http_server.workers (int, default: 1) — number of HTTP listener workers
• Also consider noting the prefixed aliases http_server.http2, http_server.buffer_max_size, http_server.buffer_chunk_size, http_server.max_connections as new canonical names
  (the un-prefixed versions remain as compatibility aliases)

pipeline/inputs/splunk.md

• Add http_server.workers (int, default: 1) — number of HTTP listener workers
• Also consider noting the prefixed aliases http_server.* as new canonical names (the un-prefixed versions remain as compatibility aliases)

pipeline/inputs/elasticsearch.md

• Add http_server.workers (int, default: 1) — number of HTTP listener workers
• Also consider noting the prefixed aliases http_server.* as new canonical names (the un-prefixed versions remain as compatibility aliases)

pipeline/inputs/prometheus-remote-write.md

• Add http_server.workers (int, default: 1) — number of HTTP listener workers
• Also consider noting the prefixed aliases http_server.* as new canonical names (the un-prefixed versions remain as compatibility aliases)