Add description of -rev parameter securityadmin.sh

[antonioazambuja]

https://opendistro.github.io/for-elasticsearch-docs/docs/security/configuration/security-admin/

In this page, not contain description of -rev parameter. This parameter makes available resolution enviroment variables before apply Opendistro Security configurations.

Example of use case enviroment variable in config.yml:

• password: ${env.LDAP_PASSWORD}

Thanks!

[aetter]

Closing out in favor of opensearch-project/documentation-website#17