XPack documentation updates

*Original comment by @jakelandis:*

Basic Auth Documentation:
-------------
LINK REDACTED
It appears that this was removed for 6.x. However, the information here is still relevant and needed.

EDIT: ^^ nvmd -> I see it just moved: https://www.elastic.co/guide/en/logstash/master/ls-security.html

Has any thought been given to use the Kibana UI instead of making HTTP calls? (The UI is slick, and the current documentation does not call out how set the HTTP basic auth to make the HTTP calls, which could be quite confusing for a newbie)

XPack configuration doc
-------------
https://www.elastic.co/guide/en/logstash/master/settings-xpack.html

Can we explicitly call out that the following username and password is for the system only and other usages, such as the elasticsearch output needs to follow the [above referenced instructions](https://www.elastic.co/guide/en/logstash/master/ls-security.html) to create the correct user
```
xpack.monitoring.elasticsearch.username and xpack.monitoring.elasticsearch.password
```

This is major nit-pick...but can we change

```
xpack.monitoring.elasticsearch.username and xpack.monitoring.elasticsearch.password
```
to
```
xpack.monitoring.elasticsearch.username
xpack.monitoring.elasticsearch.password
```

Also, while reviewing, I was fairly confused by Monitoring vs. Configuration Management Settings. Are there any subsequent pages that can be linked to/from to better understand what is Monitoring and what is Configuration Management ?

cc @dedemorton @robbavey @jordansissel