You can download the PSoC4xxx Programming Specifications from this link:
Is it useful? Absolutely. But, it all depends on what your needs are.
If you want to construct the low-level hardware to emulate the SWD interface, yes. If you plan to use Cypress supplied bootloaders, KitProg or MiniProg for download/programming, it's not really needed. It will however give you a better understanding of how the device's FLASH gets programmed from the ground up.
I doubt most developers need this document.
BTW, this document relates to PSoC4xxx FLASH downloading and FLASH programming. It's not a "how-to" write s/w.