[Doc] Add example Env var usage for hosts setting

[roaksoax]

Adds an example under hosts setting showing users how to use an environment variable with multiple white-space delimited hosts.

Fixes elastic/logstash#12661

[robbavey]

Couple of typos, and some content queries

[karenzone]

Thank you for adding this helpful example. It definitely makes the docs better. I left some suggested text inline. Note that I removed the link to the concept page. Instead, I lifted the critical part of the text and included it here.

[karenzone]

Added suggestion based on what we discussed above.

Changes addressed