Update documentation around to show that it can be configured as a list to extract subject from nested JWT claims (#11000) (#11150)

Author: opensearch-trigger-bot[bot]

_security/authentication-backends/jwt.md

@@ -120,7 +120,7 @@ Name | Description
 `signing_key` | The signing key(s) used to verify the token. If you use a symmetric key algorithm, this is the Base64-encoded shared secret. If you use an asymmetric algorithm, the algorithm contains the public key. To pass multiple keys, use a comma-separated list or enumerate the keys.
 `jwt_header` | The HTTP header in which the token is transmitted. This is typically the `Authorization` header with the `Bearer` schema,`Authorization: Bearer <token>`. Default is `Authorization`. Replacing this field with a value other than `Authorization` prevents the audit log from properly redacting the JWT header from audit messages. It is recommended that users only use  `Authorization` when using JWTs with audit logging. 
 `jwt_url_parameter` | If the token is not transmitted in the HTTP header but rather as an URL parameter, define the name of the parameter here.
-`subject_key` | The key in the JSON payload that stores the username. If not set, the [subject](https://tools.ietf.org/html/rfc7519#section-4.1.2) registered claim is used.
+`subject_key` | The key in the JSON payload that stores the username. If not set, the [subject](https://tools.ietf.org/html/rfc7519#section-4.1.2) registered claim is used. To extract a username from nested JWT claims, you can configure `subject_key` as a list.
 `roles_key` | The key in the JSON payload that stores the user's roles. The value must be a comma-separated list of roles. You can configure `roles_key` as a list to extract roles from nested JWT claims.
 `required_audience` | The name of the audience that the JWT must specify. You can set a single value (for example, `project1`) or multiple comma-separated values (for example, `project1,admin`). If you set multiple values, the JWT must have at least one required audience. This parameter corresponds to the [`aud` claim of the JWT](https://datatracker.ietf.org/doc/html/rfc7519#section-4.1.3).
 `required_issuer` | The target issuer of JWT stored in the JSON payload. This corresponds to the [`iss` claim of the JWT](https://datatracker.ietf.org/doc/html/rfc7519#section-4.1.1).

_security/authentication-backends/openid-connect.md

@@ -63,7 +63,7 @@ Name | Description
 `openid_connect_url` | The URL of your IdP where the Security plugin can find the OpenID Connect metadata/configuration settings. This URL differs between IdPs. Required when using OpenID Connect as your backend.
 `jwt_header` | The HTTP header that stores the token. Typically the `Authorization` header with the `Bearer` schema: `Authorization: Bearer <token>`. Optional. Default is `Authorization`.
 `jwt_url_parameter` | If the token is not transmitted in the HTTP header, but as an URL parameter, define the name of the parameter here. Optional.
-`subject_key` | The key in the JSON payload that stores the user's name. If not defined, the [subject](https://tools.ietf.org/html/rfc7519#section-4.1.2) registered claim is used. Most IdP providers use the `preferred_username` claim. Optional.
+`subject_key` | The key in the JSON payload that stores the user's name. If not defined, the [subject](https://tools.ietf.org/html/rfc7519#section-4.1.2) registered claim is used. Most IdP providers use the `preferred_username` claim. To extract a username from nested JWT claims, you can configure `subject_key` as a list. Optional. 
 `roles_key` | The key in the JSON payload that stores the user's roles. The value must be a comma-separated list of roles. This key is required only if you want to use roles in the JWT. You can configure `roles_key` as a list to extract roles from nested JWT claims.
 `required_audience` | The name of the audience that the JWT must specify. You can specify a single value (for example, `project1`) or multiple comma-separated values (for example, `project1,admin`). If you specify multiple values, the JWT must have at least one required audience. This parameter corresponds to the [`aud` claim of the JWT](https://datatracker.ietf.org/doc/html/rfc7519#section-4.1.3).
 `required_issuer` | The target issuer of the JWT stored in the JSON payload. This corresponds to the [`iss` claim of the JWT](https://datatracker.ietf.org/doc/html/rfc7519#section-4.1.1).