Add note about exisiting log groups

Author: humivo

src/docs/getting-started/cloudwatch-metrics.mdx

@@ -76,6 +76,8 @@ Exporter can also be customized in the configuration and be set to the following
 
 0 (Never Expire), 1, 3, 5, 7, 14, 30, 60, 90, 120, 150, 180, 365, 400, 545, 731, 1827, 2192, 2557, 2922, 3288, or 3653.
 
+Log Retention can only be set this way for newly created Cloudwatch log groups.  This feature will not update log retention for exisiting Cloudwatch log groups.
+
 The available dimension roll-up options in EMF exporter,
 * "ZeroAndSingleDimensionRollup" - Enable both zero dimension rollup and single dimension rollup
 * "SingleDimensionRollupOnly" - Enable single dimension rollup