no default hash partition

Given the discussion starting at
https://postgr.es/m/CAFjFpRdBiQjZm8sG9+s0x8Re-afHds6MFLgGuw0wVUNLGrVOQg@mail.gmail.com
we don't have default-partition support with the hash partitioning
scheme.  That seems a reasonable outcome, but I think we should have a
comment about it (I had to search the reason for this restriction in the
hash-partitioning patch set).  How about the attached?  Does anyone see
a reason to make this more verbose, and if so to what?

... unless somebody wants to argue that we should have the feature; if
so please share your patch.

Thanks

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services