I understand your confusion, "power interface clock" may sound confusing at the first sight. However, it is expected, that you've read at least the first few chapters of the RM before starting working with the chips.
So, chapter 2.1.6 BusMatrix says:
After each device reset, all peripheral clocks are disabled (except for the SRAM1/2 and
Flash memory interface). Before using a peripheral you have to enable its clock in the
RCC_AHBxENR and the RCC_APBxENR registers.
"Peripheral" here refers to the modules (sometimes called IP), represented by the individual chapters later on in the RM, e.g. TIM, UART, etc. PWR is one of those peripherals, and you can't access its registers until you enable its clock.
Regardless of this, analog/power circuitry is working after reset according to the reset values of the PWR registers (see description of individual bits and bitfields in those registers in the PWR chapter).
JW
PS. Please don't attach files which are commonly available through a link, post the link instead. And you don't need to do even that for obvious resources - the RM is the basic tool used by all STM32 developers daily, so it's enough to refer to RM number (RM0351 in this case), every reader of this forum knows what does that mean.
