[APM] Custom links: Update copy in the Filters section

[formgeist]

Summary

Add copy to the Filters section in the flyout to indicate how those filters work. Right now it's state in the introduction, but it's not very visible.

Solution

• Change the flyout introduction:
  Links will be available in the context of transaction details throughout the APM app. You can create an unlimited number of links. You can refer to dynamic variables by using any of the transaction metadata to fill in your URLs. Learn more about it in the docs.
• Add paragraph after the Filters label: Use the filter options to scope them to only appear for specific services.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[cauemarcondes]

@formgeist we still don't have the link to Learn more about it in the docs?

[formgeist]

@formgeist we still don't have the link to Learn more about it in the docs?

I pinged Brandon in the issue https://github.com/elastic/observability-dev/issues/650#issuecomment-600059955

[bmorelli25]

Here's where custom link docs will live:

https://www.elastic.co/guide/en/kibana/<VERSION>/custom-links.html

It might make sense to change the link text to reference the fact that we also have examples in the documentation. Something like, "More information, including examples, are available in the [documentation]." Or, for more fun, "Not sure where to start? Examples for creating custom email links, GitHub issues, and more, are available in the [docs].