@Gopikrishnan Bahuleyan Pillai We can confirm that, when routing to Event Hub, the partition key stays deviceid.
Regarding the documentation, We have informed docs team to update the documentation.
To understand more about the partitions, please see Partition key and Partitions
Do let us know if you have any further queries.
------------
If an answer is helpful, please "Accept answer" or "Up-Vote" for the same which might be beneficial to other community members reading this thread.