Camera: Improve logical camera RAW capture docs

- Clarify raw capture behavior for logical camera with backward
compatible approach and MultiResolutionImageReader.
- Some logical camera devices may have identical physical cameras with
different focal length. Allow such logical camera to switch physical
cameras when outputing raw.

Bug: 182217158
Test: Build
Change-Id: I64458683f4eb8be6fc1e35270a8c22864d962be9

core/java/android/hardware/camera2/CameraMetadata.java

@@ -991,13 +991,27 @@ public abstract class CameraMetadata<TKey> {
      * camera's crop region is set to maximum size, the FOV of the physical streams for the
      * ultrawide lens will be the same as the logical stream, by making the crop region
      * smaller than its active array size to compensate for the smaller focal length.</p>
-     * <p>Even if the underlying physical cameras have different RAW characteristics (such as
-     * size or CFA pattern), a logical camera can still advertise RAW capability. In this
-     * case, when the application configures a RAW stream, the camera device will make sure
-     * the active physical camera will remain active to ensure consistent RAW output
-     * behavior, and not switch to other physical cameras.</p>
+     * <p>There are two ways for the application to capture RAW images from a logical camera
+     * with RAW capability:</p>
+     * <ul>
+     * <li>Because the underlying physical cameras may have different RAW capabilities (such
+     * as resolution or CFA pattern), to maintain backward compatibility, when a RAW stream
+     * is configured, the camera device makes sure the default active physical camera remains
+     * active and does not switch to other physical cameras. (One exception is that, if the
+     * logical camera consists of identical image sensors and advertises multiple focalLength
+     * due to different lenses, the camera device may generate RAW images from different
+     * physical cameras based on the focalLength being set by the application.) This
+     * backward-compatible approach usually results in loss of optical zoom, to telephoto
+     * lens or to ultrawide lens.</li>
+     * <li>Alternatively, to take advantage of the full zoomRatio range of the logical camera,
+     * the application should use {@link android.hardware.camera2.MultiResolutionImageReader }
+     * to capture RAW images from the currently active physical camera. Because different
+     * physical camera may have different RAW characteristics, the application needs to use
+     * the characteristics and result metadata of the active physical camera for the
+     * relevant RAW metadata.</li>
+     * </ul>
      * <p>The capture request and result metadata tags required for backward compatible camera
-     * functionalities will be solely based on the logical camera capabiltity. On the other
+     * functionalities will be solely based on the logical camera capability. On the other
      * hand, the use of manual capture controls (sensor or post-processing) with a
      * logical camera may result in unexpected behavior when the HAL decides to switch
      * between physical cameras with different characteristics under the hood. For example,