Add docs to cover recent LAYOUT_all lint checks (#19393)

docs/hardware_keyboard_guidelines.md

@@ -208,6 +208,8 @@ As an example, if you have a 60% PCB that supports ANSI and ISO you might define
 | LAYOUT_ansi | default_ansi | An ANSI layout |
 | LAYOUT_iso | default_iso | An ISO layout |
 
+?> Providing only `LAYOUT_all` is invalid - especially when implementing the additional layouts within 3rd party tooling.
+
 ## Image/Hardware Files
 
 In an effort to keep the repo size down we're no longer accepting binary files of any format, with few exceptions. Hosting them elsewhere (such as <https://imgur.com>) and linking them in the `readme.md` is preferred.

docs/pr_checklist.md

@@ -55,6 +55,8 @@ https://github.com/qmk/qmk_firmware/pulls?q=is%3Apr+is%3Aclosed+label%3Akeyboard
         - `layout` definitions should include matrix positions, so that `LAYOUT` macros can be generated at build time
             - should use standard definitions if applicable
             - use the Community Layout macro names where they apply (preferred above `LAYOUT`/`LAYOUT_all`)
+            - use of `LAYOUT_all` is only valid when providing additional layout macros
+              - providing only `LAYOUT_all` is invalid - especially when implementing the additional layouts within 3rd party tooling
 - `readme.md`
     - standard template should be present -- [link to template](https://github.com/qmk/qmk_firmware/blob/master/data/templates/keyboard/readme.md)
     - flash command is present, and has `:flash` at end