DOCSP-27027 unique and nonUnique indexes (#104)

Author: Dave Cuthbert

source/reference/limitations.txt

@@ -62,6 +62,8 @@ General Limitations
   downgrade, you should drop all non-system databases in the destination
   cluster before starting the new binary. Syncing operations will
   restart from the beginning when you start the new processes.
+- You cannot sync a collection that has a unique index and a non-unique
+  index defined on the same field(s).
 
 MongoDB Community Edition
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