Documentation: leds: Add "rgb:status" path

The path /sys/class/leds/rgb:status is already widely used with the
qcom-lpg driver and others. Document it.
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lore.kernel.org/r/20230414-pmi632-v3-3-079d2cada699@z3ntu.xyzSigned-off-by: Lee Jones <lee@kernel.org>

Documentation/leds/well-known-leds.txt

@@ -58,6 +58,7 @@ LEDs on notebook body, indicating that sound input / output is muted.
 
 * System notification
 
+Good: "rgb:status"
 Legacy: "status-led:{red,green,blue}" (Motorola Droid 4)
 Legacy: "lp5523:{r,g,b}" (Nokia N900)