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/libcamera/camera_manager.cpp

@@ -367,7 +367,7 @@ std::shared_ptr<Camera> CameraManager::get(const std::string &id)
  * \brief Retrieve a camera based on device number
  * \param[in] devnum Device number of camera to get
  *
- * This method is meant solely for the use of the V4L2 compatibility
+ * This function is meant solely for the use of the V4L2 compatibility
  * layer, to map device nodes to Camera instances. Applications shall
  * not use it and shall instead retrieve cameras by name.
  *