MMS OnPrem 1.5: Document LDAP Auth for web server <-> LDAP

The procedure for configuring MMS to talk to a backing MongoDB using LDAP is very similar to MONGODB-CR authentication. The username and password are supplied in the `mongoUri` properties in the properties file. Encrypted credentials are supported in the same way as MONGODB-CR. Adding `authMechanism=PLAIN&authSource=$external` to the URI specifies that these credentials are for LDAP authentication.

The following example is in the properties file shipped with MMS:

1. mongo.mongoUri=mongodb://user:password@server:port/?authMechanism=PLAIN&authSource=$external