libcamera: controls: Improve documentation for ExposureTime and AnalogueGain

Setting these controls "fixes" them and the AE may not change them;
setting them back to zero returns them to the control of the AE
algorithm.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/control_ids.yaml

@@ -125,8 +125,18 @@ controls:
         Exposure time (shutter speed) for the frame applied in the sensor
         device. This value is specified in micro-seconds.
 
+        Setting this value means that it is now fixed and the AE algorithm may
+        not change it. Setting it back to zero returns it to the control of the
+        AE algorithm.
+
         \sa AnalogueGain AeEnable
 
+        \todo Document the interactions between AeEnable and setting a fixed
+        value for this control. Consider interactions with other AE features,
+        such as aperture and aperture/shutter priority mode, and decide if
+        control of which features should be automatically adjusted shouldn't
+        better be handled through a separate AE mode control.
+
   - AnalogueGain:
       type: float
       description: |
@@ -134,8 +144,18 @@ controls:
         The value of the control specifies the gain multiplier applied to all
         colour channels. This value cannot be lower than 1.0.
 
+        Setting this value means that it is now fixed and the AE algorithm may
+        not change it. Setting it back to zero returns it to the control of the
+        AE algorithm.
+
         \sa ExposureTime AeEnable
 
+        \todo Document the interactions between AeEnable and setting a fixed
+        value for this control. Consider interactions with other AE features,
+        such as aperture and aperture/shutter priority mode, and decide if
+        control of which features should be automatically adjusted shouldn't
+        better be handled through a separate AE mode control.
+
   - Brightness:
       type: float
       description: |