Looks like part of the info is missing here for the C docs (notably the 50fps max and notes on setting it to unlocked):
Sets the nominal refresh rate in frames per second. Default is 20 fps, the maximum rate supported by the hardware for full-frame updates.
Sets the nominal refresh rate in frames per second. The default is 30 fps, which is a recommended figure that balances animation smoothness with performance and power considerations. Maximum is 50 fps.
If rate is 0, playdate.update() is called as soon as a frame buffer is available. Since the display refreshes line-by-line, and unchanged lines aren’t sent to the display, the update cycle will be faster than 30 times a second but at an indeterminate rate.