docs(rc_update): clarify that RC_CHAN_CNT gates RC input validity (#27443)

RC_CHAN_CNT is not "only meant for ground station use" — rc_update
reads it to compute _rc_calibrated, which gates
manual_control_input.valid. A value of 0 causes the RC manual
control input to be marked invalid and silently ignored by the
manual control selector, so flight mode switching and stick inputs
from RC stop working.

Tighten the short to mention calibration state and rewrite the long
to state the actual rule the firmware enforces.

Refs PX4#27439

Signed-off-by: Jacob Dahl <dahl.jakejacob@gmail.com>

src/modules/rc_update/params.yaml

@@ -688,11 +688,10 @@ parameters:
       max: 1
     RC_CHAN_CNT:
       description:
-        short: RC channel count
+        short: Calibrated RC channel count
         long: |-
-          This parameter is used by Ground Station software to save the number
-          of channels which were used during RC calibration. It is only meant
-          for ground station use.
+          Number of channels detected during RC calibration. Must be non-zero
+          for RC manual control input to be accepted.
       type: int32
       default: 0
       min: 0