I'm having trouble coming up with an official reference. (I'm not surprised, either...)
This Wiki article enumerates a list of attributes required for objects to be "considered for sync". Quoth the article:
- member (applies only to groups)
- samAccountName (applies only to users)
- alias (applies only to groups and contacts)
- displayName (for groups with an mail or proxyAddresses attribute populated)
This seems to be further-corroborated by a RMS team blog posting identifying the minimum attribute set necessary for Azure RMS. Obviously, you're not talking about Azure RMS, but I think it's reasonable to consider these attributes as at least partially overlapping with the list you're looking for.
Finally, I found this community post wherein the poster talks about a minimum attribute-set that they came up with.
I'd like to say that I'm shocked that finding an official source for this information is difficult.