in_splunk: Add remote_addr related parameters' descriptions

Signed-off-by: Hiroshi Hatake <hiroshi@chronosphere.io>

Author: cosmo0920

pipeline/inputs/splunk.md

@@ -17,6 +17,8 @@ This plugin uses the following configuration parameters:
 | `splunk_token`             | Specify a Splunk token for HTTP HEC authentication. If multiple tokens are specified (with commas and no spaces), usage will be divided across each of the tokens. | _none_          |
 | `store_token_in_metadata`  | Store Splunk HEC tokens in the Fluent Bit metadata. If set to `false`, tokens will be stored as normal key-value pairs in the record data.                         | `true`          |
 | `splunk_token_key`         | Use the specified key for storing the Splunk token for HTTP HEC. Use only when `store_token_in_metadata` is `false`.                                               | `@splunk_token` |
+| `add_remote_addr`          | Inject the client remote address into the record. The value is extracted from the X-Forwarded-For header if present, otherwise from the connection address.        | `false`         |
+| `remote_addr_key`          | Set the record key used to store the remote address when `add_remote_addr` is enabled.                                                                             | `remote_addr`   |
 | `Threaded`                 | Indicates whether to run this input in its own [thread](../../administration/multithreading.md#inputs).                                                            | `false`         |
 
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