[DOCS] Clean up APM UI docs (elastic#169269)

Closes elastic/observability-docs#3292

## Summary

Updates documentation related to the APM UI based on issues found during
the Observability docs bug bash last week.

### Checklist

Delete any items that are not applicable to this PR.

- [x] @colleenmcginnis
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials:
- [x] Clarify that distributed traces aren't the only colorful traces in
[Trace sample
timeline](https://www.elastic.co/guide/en/kibana/current/spans.html#distributed-tracing)
(elastic@b707c39)
- [x] Clarify the implications of the traces table only showing root
transactions in
[Traces](https://www.elastic.co/guide/en/kibana/current/traces.html)
(elastic@c1de678)
- [x] Add beta admonition to APM
[Infrastucture](https://www.elastic.co/guide/en/kibana/current/infrastructure.html)
page
(elastic@d3bc4a9)
- [x] @bmorelli25 review

docs/apm/infrastructure.asciidoc

@@ -2,6 +2,8 @@
 [[infrastructure]]
 === Infrastructure
 
+beta::[]
+
 The *Infrastructure* tab provides information about the containers, pods, and hosts,
 that the selected service is linked to.
 

docs/apm/spans.asciidoc

@@ -43,8 +43,9 @@ For example, quickly view:
 [[distributed-tracing]]
 ==== Distributed tracing
 
-If your trace sample timeline is colorful, it's indicative of a distributed trace.
-Services in a distributed trace are separated by color and listed in the order they occur.
+When a trace travels through multiple services it is known as a _distributed trace_.
+In APM, the colors in a distributed trace represent different services and
+are listed in the order they occur.
 
 [role="screenshot"]
 image::apm/images/apm-services-trace.png[Example of distributed trace colors in the APM app in Kibana]

docs/apm/traces.asciidoc

@@ -17,6 +17,10 @@ it's the collective amount of pain a specific endpoint is causing your users.
 If there's a particular endpoint you're worried about, select it to view its
 <<transaction-details,transaction details>>.
 
+You can also use queries to filter and search the transactions shown on this page.
+Note that only properties available on root transactions are searchable.
+For example, you can't search for `label.tier: 'high'`, as that field is only available on non-root transactions.
+
 [role="screenshot"]
 image::apm/images/apm-traces.png[Example view of the Traces overview in APM app in Kibana]