The function prototype and constant definitions in the post are cut from a CCS header file of dsPIC or similar device. setup_motor_pwm() is a CCS PCD (16-Bit PIC compiler) built-in function. It's documented in the PCD online help for V4 and V5 compilers.
Apart from annoying cross-posting, the OP's message is questionable in several regards. What has the usability of a C compiler to do with the existence of built-in functions for specific purposes? Many professional compilers come without any application specific built-in functions.
I'm tempted to quote a saying about a workman and his tools. It would be better to say "I miss a documentation", or "I don't understand this point".
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I forgot to mention that there has been a mixed-up online documentation in some recent CCS versions, at least V5.019. The bug has been fixed in V5.020.