update aggregation limitations

Author: bhillmer

infrastructure/metrics-pipeline/metrics-pipeline-intro.rst

@@ -167,20 +167,16 @@ MPM is not available for the following types of metrics:
 Aggregation rules limitations
 --------------------------------------------------------------------------------
 
-You can only create aggregation rules using your metrics' dimensions. Aggregation using custom properties or tags is not supported. For more information on each type of metadata, refer to :ref:`metrics-dimensions-mts`.
+* You can only create aggregation rules using your metrics dimensions. Aggregation using custom properties or tags is not supported. For more information on each type of metadata, refer to :ref:`metrics-dimensions-mts`.
+* New aggregation rules are applied to new MTS only. Existing MTS are only used as a reference to create the rule and display the projected outcome.
 
 Histogram metrics limitations
 --------------------------------------------------------------------------------
 
-You cannot archive or aggregate histogram metrics. By default, they are routed to the real-time tier, and you can drop them with rules as well.
+You can't archive or aggregate histogram metrics. By default, they are routed to the real-time tier, and you can drop them with rules as well.
 
 .. _metrics-pipeline-intro-more:
 
-Aggregation rules limitations
---------------------------------------------------------------------------------
-
-You can only create aggregation rules using your metrics' dimensions. Aggregation using custom properties or tags is not supported. For more information on each type of metadata, refer to :ref:`metrics-dimensions-mts`.
-
 Learn more
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