Improve changelog

Author: alex-kulakov

ChangeLog/6.0.13_dev.txt

@@ -1,4 +1,5 @@
 [main] Fixed certain cases of bad translation of casts via 'as' operator in LINQ queries
 [main] Addressed certain issues of translation connected with comparison with local entity instace within LINQ queries
 [main] Fixed rare issues of incorrect translation of filtered index expressions including conditional expressions
+[main] Join/LeftJoin is denied to have the same expression instance for both inner/outer selector.
 [postgresql] Fixed issue of incorrect translation of contitional expressions including comparison with nullable fields