Merge pull request #496 from prometheus/beorn7/doc-improve

Slightly improve wording about series cardinality

content/docs/practices/naming.md

@@ -55,8 +55,8 @@ Use labels to differentiate the characteristics of the thing that is being measu
 Do not put the label names in the metric name, as this introduces redundancy
 and will cause confusion if the respective labels are aggregated away.
 
-CAUTION: **CAUTION:** Remember that every unique key-value label pair
-represents a new time series, which can dramatically increase the amount of
-data stored. Do not use labels to store dimensions with high cardinality (many
-different label values), such as user IDs, email addresses, or other unbounded
-sets of values.
+CAUTION: **CAUTION:** Remember that every unique combination of key-value label
+pairs represents a new time series, which can dramatically increase the amount
+of data stored. Do not use labels to store dimensions with high cardinality
+(many different label values), such as user IDs, email addresses, or other
+unbounded sets of values.