fix doc uniqueness group

related with #1633 #1589

Given a measure (identified by an apikey) there is no way to identify group related if apikey is not unique for all services and subservices.

My comment #1633 (comment) was  taken directly from #1592 (comment)

doc/api.md

@@ -165,7 +165,7 @@ parameters defined at device level in database, the parameters are inherit from
 
 ### Uniqueness of groups and devices
 
-Group uniqueness is defined by the combination of: service, subservice, resource and apikey
+Group uniqueness is defined by the combination of: resource and apikey.
 
 Device uniqueness is defined by the combination of: service, subservice, device_id and apikey. Note that several devices
 with the same device_id are allowed in the same service and subservice as long as their apikeys are different.