Document limitations of Servlet Filter observations

This commit documents the fact that the Servlet Filter based
observations for MVC applications is limited by the Servlet Filter
contract in the first place. All processing and logging that happens
outside of the scope of the filter is not observed.
Log statements from the catalina engine (in the case of Tomcat), or any
container-specific infrastructure, is not covered by the
instrumentation.

Closes gh-29398

Author: bclozel

framework-docs/modules/ROOT/pages/integration/observability.adoc

@@ -97,6 +97,10 @@ You can, at any point during request processing, set the error field on the `Obs
 
 include-code::./UserController[]
 
+NOTE: Because the instrumentation is done at the Servlet Filter level, the observation scope only covers the filters ordered after this one as well as the handling of the request.
+Typically, the Servlet container error handling is done at a lower level and won't have any active observation nor span.
+For this use case, a container-specific implementation is required, such as a `org.apache.catalina.Valve` for Tomcat; this is outside of the scope of this project.
+
 By default, the following `KeyValues` are created:
 
 .Low cardinality Keys