Add some clarity regarding new board definitions (#16018)

docs/pr_checklist.md

@@ -110,6 +110,8 @@ Also, specific to ChibiOS:
     - a lot of the time, an equivalent Nucleo board can be used with a different flash size or slightly different model in the same family
         - example: For an STM32L082KZ, given the similarity to an STM32L073RZ, you can use `BOARD = ST_NUCLEO64_L073RZ` in rules.mk
     - QMK is migrating to not having custom board definitions if at all possible, due to the ongoing maintenance burden when upgrading ChibiOS
+- New board definitions must not be embedded in a keyboard PR
+    - See _Core PRs_ below for the procedure for adding a new board to QMK
 - if a board definition is unavoidable, `board.c` must have a standard `__early_init()` (as per normal ChibiOS board defs) and an empty `boardInit()`:
     - see Arm/ChibiOS [early initialization](https://docs.qmk.fm/#/platformdev_chibios_earlyinit?id=board-init)
     - `__early_init()` should be replaced by either `early_hardware_init_pre()` or `early_hardware_init_post()` as appropriate
@@ -118,7 +120,7 @@ Also, specific to ChibiOS:
 ## Core PRs
 
 - must now target `develop` branch, which will subsequently be merged back to `master` on the breaking changes timeline
-- any support for new hardware now requires a corresponding test board under `keyboards/handwired/onekey`
+- any new boards adding support for new hardware now requires a corresponding test board under `keyboards/handwired/onekey`
     - for new MCUs, a new "child" keyboard should be added that targets your newly-added MCU, so that builds can be verified
     - for new hardware support such as display panels, core-side matrix implementations, or other peripherals, an associated keymap should be provided
     - if an existing keymap exists that can leverage this functionality this may not be required (e.g. a new RGB driver chip, supported by the `rgb` keymap) -- consult with the QMK Collaborators on Discord to determine if there is sufficient overlap already