libcamera: Rename 'method' to 'function'

Usage of 'method' to refer to member functions comes from Java. The C++
standard uses the term 'function' only. Replace 'method' with 'function'
or 'member function' through the whole code base and documentation.
While at it, fix two typos (s/backeng/backend/).

The BoundMethod and Object::invokeMethod() are left as-is here, and will
be addressed separately.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Acked-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/android/camera_device.cpp

@@ -305,7 +305,7 @@ std::unique_ptr<CameraDevice> CameraDevice::create(unsigned int id,
  * cameraConfigData is optional for external camera devices and can be
  * nullptr.
  *
- * This method is called before the camera device is opened.
+ * This function is called before the camera device is opened.
  */
 int CameraDevice::initialize(const CameraConfigData *cameraConfigData)
 {