Doc: Improve documentation on variable substitution in monitoring and management docs

[robbavey]

Since #12051 was committed, xpack.managment.elasticsearch.hosts and xpack.monitoring.elasticsearch.hosts support functionality to allow multiple elasticsearch hosts to be specified in a single ENV, by using space delimited values. We should document this to clarify this point, and provide examples of its use.

[karenzone]

Thoughts on placement:

• The plugin use case is documented here: https://www.elastic.co/guide/en/logstash/current/plugin-concepts.html#space-delimited-uris-in-list-params
• As a NOTE in the description for xpack.monitoring.elasticsearch.hosts setting in General monitoring settings
• As a NOTE in the descirption for xpack.managment.elasticsearch.hosts setting in Configuration Management Settings in Logstash
• Written from a broader, generally application section in: Tips and best practices.
  • Idea: Let's consider adding the content here, and giving a brief description of functionality and linking to it from other locations.

Tangentially related: #13419