Update synthetics-private-location.asciidoc (#4234) (#4265)

As we have had multiple support cases where customers tried to run browser-based checks via different installation procedures and even support engineers struggled to see the note above my change, I think this is important to reduce the number of cases with this.

(cherry picked from commit 97e4252)

Co-authored-by: Jan Doberstein <[email protected]>

Author: mergify[bot]

docs/en/observability/synthetics-private-location.asciidoc

@@ -75,6 +75,11 @@ For monitors running on {private-location}s, you _must_ use the `elastic-agent-c
 Docker image to create a self-hosted {agent} node. The standard {ecloud} or self-hosted
 {agent} will not work.
 
+[IMPORTANT]
+====
+The `elastic-agent-complete` Docker image is the only way to have all available options that you see in the {ecloud}.
+====
+
 ifeval::["{release-state}"=="unreleased"]
 
 Version {version} has not yet been released.