Add security.ldap.validateLDAPServerConfig to Server documentation

security.ldap.validateLDAPServerConfig is a new MongoDB Server configuration setting used by OpsManager 3.6.5, new for MongoDB 3.4.14 and 3.6.3. However, this config setting does not appear anywhere in the MongoDB Server documentation, and only appears in the changelog and release notes for OpsManager and the Automation Agent. Might be a good idea to add it to the server documentation as well.