Documentation for twentynm_iopll

Thanks, @RichardTanSY_Altera.

Yes, I had already found the component declaration, but without documentation that's not particularly useful.

As to the IP, I've now obtained clarification on the IP's behavior that I was observing. The IP is reportedly operating per its design intent. The apparent discrepancy was a matter of misunderstanding / incorrect assumptions about the IOPLL's operating characteristics when its refclk input is fed by a GCLK, a supported topology but whose compensation characteristics were not explicitly specified in the documentation for the IP either.

A general plea to Intel/Altera, not only on this particular issue: Please make more comprehensive documentation available to us end users. It will save everyone a lot of time and headaches, and reduce Intel's customer support burden.

Thanks,
-Roee