Refine PR text instructions (#14397)

Author: koppor

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,7 @@
 Closes _____
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-<!-- In about one to three sentences, describe the changes you have made: what, where, why, ... (REPLACE THIS LINE) -->
+<!-- In about one to three sentences, describe the changes you have made: what, where, why, ... Summarize changes and DO NOT list modified classes one-by-one. (REPLACE THIS LINE) -->
 
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