Docs: X-Pack LDAP Parameter "realms" missing from documentation

[Jymit]

Also making mention of elastic/docs#244 here FAO documentation teams.
cc @MorrieAtElastic

1. In this document we pertain to xpack.security.authc.active_directory.files.role_mapping however this is broken and misleading. https://www.elastic.co/guide/en/x-pack/current/mapping-roles.html

2. Here is more so correct xpack.security.authc.realms. https://www.elastic.co/guide/en/x-pack/current/active-directory-realm.html#_configuring_an_active_directory_realm

Please see this as an ask to update doc 1 so there are no discrepancies between the two.
Thanks!

[elasticmachine]

Pinging @elastic/es-security

[jaymode]

@lcawl can you take a look at this?