The W65C02 data sheet does explain it (p. 9):
3.4 Interrupt Request (IRQB)
The Interrupt Request (IRQB) input signal is used to request that an interrupt sequence be initiated. The program
counter (PC) and Processor Status Register (P) are pushed onto the stack and the IRQB disable (I) flag is set to a "1"
disabling further interrupts before ...