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 <[e-mail]>
Acked-by: Dave Airlie <[e-mail]>
[jc: fixed "no trailing newline"]
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/driver-api/firmware/firmware-usage-guidelines.rst

···
   deprecating old major versions, then this should only be done as a
   last option, and be stated clearly in all communications.
 
+* Firmware files that affect the User API (UAPI) shall not introduce
+  changes that break existing userspace programs. Updates to such firmware
+  must ensure backward compatibility with existing userspace applications.
+  This includes maintaining consistent interfaces and behaviors that
+  userspace programs rely on.