docs: driver-api: firmware: clarify userspace requirements

docs: driver-api: firmware: clarify userspace requirements

The guidelines mention that firmware updates can't break the kernel,
but it doesn't state directly that they can't break userspace programs.
Make it explicit that firmware updates cannot break UAPI.

Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
[jc: fixed "no trailing newline"]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250314100137.2972355-1-jacek.lawrynowicz@linux.intel.com