Update rare_term version skips, fix SetBackedScalingCuckooFilter javadoc

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/280_rare_terms.yml

@@ -1,6 +1,6 @@
 setup:
   - skip:
-      version: " - 8.0.0" # TODO change this after backport
+      version: " - 7.2.99"
       reason: RareTerms added in 7.3.0
   - do:
       indices.create:

server/src/main/java/org/elasticsearch/common/util/SetBackedScalingCuckooFilter.java

@@ -38,7 +38,7 @@ import java.util.function.Consumer;
 /**
  * An approximate set membership datastructure that scales as more unique values are inserted.
  * Can definitively say if a member does not exist (no false negatives), but may say an item exists
- * when it does not (has false negatives).  Similar in usage to a Bloom Filter.
+ * when it does not (has false positives).  Similar in usage to a Bloom Filter.
  *
  * Internally, the datastructure maintains a Set of hashes up to a specified threshold.  This provides
  * 100% accurate membership queries.