Welcome to Arduino.SE. The documentation that you seek on
PINC would not be documented under Arduino, this is as these are directly related to the ATMega328 (AVR Atmel micrcontroller) which is on the Arduino, they are registers which are inside the microcontroller and
PINC are mnemonics.
For the ATMega328 you will find the documentation to these registers in the datasheet (a good read with coffee), or in tutorials written by others explaining this information in an easy to grasp way.
PCIFR register is the Pin Change interrupt flag register(description on pg 73 of linked datasheet), this holds the flag bit to tell the microcontroller that there was an interrupt and runs the
ISR() routine that one has set out. Read this post by Nick Gammon on Pin Change Interrupts, and then read all of the post to get a better idea of interrupts in general.
PINC is for the inputs to the microcontroller, when one uses
staus = digitalRead(A0); they are in effect using a wrapped up version of bitwise math and bit manipulation when simplified to
status = PINC & (1 << PC0); thus
PINx (x is for the register name ABCD) represents the state of the pin, either HIGH(1, 5V, true) or LOW(0, 0V, False). Borrowing from the below linked tutorial:
Another nice description of the input ports in this tutorial.
From the image you see that moving a bit to a position and then bitwise
& that with
PINx you will have a value returned if there is a 1 in that position.
(1 << PC0) 0b00000001
status = 0b00000001
The above shows how you only get the state of the first input and how the 4th input is ignored as it was not needed. Also worth noting PIN is the register description and is not the same as a physical pin that sticks out the IC's body.