[docs/collector] Clarify stance with regards to third-party instrumentation

Author: mx-psi

content/en/docs/collector/internal-telemetry.md

@@ -293,7 +293,9 @@ There are currently no metrics specific to `normal` verbosity.
 {{% alert title="Note" color="info" %}} The `http_` and `rpc_` metrics come from
 instrumentation libraries. Their original names use dots (`.`), but when
 exposing internal metrics with Prometheus, they are translated to use
-underscores (`_`) to match Prometheus' naming constraints.
+underscores (`_`) to match Prometheus' naming constraints. These metrics are not
+covered by the maturity levels below since they are not under the Collector SIG
+control.
 
 The `otelcol_processor_batch_` metrics are unique to the `batchprocessor`.
 
@@ -317,6 +319,10 @@ The Collector logs the following internal events:
 
 ## Telemetry maturity levels
 
+The Collector telemetry levels apply to all first-party telemetry produced by
+the Collector. Third-party libraries, including those of OpenTelemetry Go, are
+not covered by these maturity levels.
+
 ### Traces
 
 Tracing instrumentation is still under active development, and changes might be
@@ -326,10 +332,13 @@ guarantees of backwards compatibility for tracing instrumentation.
 
 ### Metrics
 
-The Collector's metrics follow a four-stage lifecycle:
+The Collector's first-party metrics follow a four-stage lifecycle:
 
 > Alpha metric → Stable metric → Deprecated metric → Deleted metric
 
+Third-party metrics, including those generated by OpenTelemetry Go
+instrumentation libraries, are not covered by these maturity levels.
+
 #### Alpha
 
 Alpha metrics have no stability guarantees. These metrics can be modified or