I found the document easy to read and understandable.
The last GPON stuff I did was in 2010, so I don't really know the details of the technology anymore.
The interspersing of text into the YANG-tree output is an interesting way to do things. I was concerned as I read that this might mean that description in the YANG itself might be weak, and I found this to be the case. I don't have a good answer as to whether detailed text in the YANG module is better or worse.
Nits:
Section 4.2 has some odd formatting for the definition list, which I'm sure the RPC will clean up.
See: https://mailarchive.ietf.org/arch/msg/ccamp/xixPovLC47O-1d_tN66w68_e1ho/