[Docs] clarify plugin config / scrape config documentation

[erSitzt]

Hi,

the documentation states that the plugin need to be installed on every node that will be scraped, the example prometheus job config lists multiple nodes to be scraped.
On the other hand there is a config option to include node metrics not only for the node that is scraped... this leads to some questions for me:

• Do i need to scrape all nodes or is it sufficient to enable that option
• If i scrape all nodes, do they all report the identical redundand opensearch metrics 2-x times in addition to the nodes metrics ?
• Whats the best practice for scraping a 3-x node cluster with all node metrics ?

Thanks :)