Merge pull request #1694 from telefonicaid/AlvaroVega-patch-1

Update api.md

Author: fgalan

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: resource and apikey.
+Group uniqueness is defined by the combination of: resource and apikey. This is so because 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.
 
 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.