Why Atmega328P has no PortA
When a chip manufacturer designs a family of chips they will often start with an all singing, all dancing design with all the bells and whistles.  They will then create cut-down versions of the chip by removing portions of the design, or sometimes just not connecting pins up.

The internals of the 328P will be based on a larger chip which does have a port A (as well as other ports).  The removal of port A as opposed to another port would have had two considerations:


What functions are shared with which port, and which can we do away with
How easy would it be to move any extra functions to other pins
How easy would it be to remove the port from the design


I would surmise that port A had few extra functions shared with its pins, and the port was simple to remove from the design and leave the chip suitable for use in a low pin count package.

And yes, the port A reference in the ADC section should read port C.