Update installing-stack-demo-self.asciidoc to mitigate error-handling issue. update variable on tail command

[rich-cabrera]

Adding an Important clarification. The enrollment script fails without proper error handling if the second node is not able to access the first node. the elasticsearch.yml ends up modified and the enrollment script is unable to be reran without undoing all the changes. Reminder added to ensure network access is available before proceeding.

second. the tail command is incorrect unless the cluster.name is also elasticsearch-demo. this value defaults to cluster.name

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[rich-cabrera]

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[shainaraskas]

hi @rich-cabrera! we're releasing our new docs system tomorrow so I included your change in my PR here.

we can ship this, but 8.18 is also releasing tomorrow so we'll need to try to forward-port your changes. haven't tried forward porting in a while so we can see if it works.

[leemthompo]

@elasticmachine test this

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run docs-build