[chore][processor/cumulativetodelta] clarify handling of delta metrics (open-telemetry#44230)

Signed-off-by: odubajDT <[email protected]>

Author: odubajDT

processor/cumulativetodeltaprocessor/README.md

@@ -16,7 +16,7 @@
 
 ## Description
 
-The cumulative to delta processor (`cumulativetodeltaprocessor`) converts monotonic, cumulative sum and histogram metrics to monotonic, delta metrics. Non-monotonic sums and exponential histograms are excluded.
+The cumulative to delta processor (`cumulativetodeltaprocessor`) converts monotonic, cumulative sum and histogram metrics to monotonic, delta metrics. Non-monotonic sums and exponential histograms are excluded. Delta metrics are excluded from any conversion and forwarded without changes.
 
 ## Configuration