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/media_device.cpp

@@ -134,7 +134,7 @@ void MediaDevice::release()
  * they provide at all times, while still allowing an instance to lock a
  * resource while it prepares to actively use a camera from the resource.
  *
- * This method shall not be called from a pipeline handler implementation
+ * This function shall not be called from a pipeline handler implementation
  * directly, as the base PipelineHandler implementation handles this on the
  * behalf of the specified implementation.
  *
@@ -161,7 +161,7 @@ bool MediaDevice::lock()
 /**
  * \brief Unlock the device and free it for use for libcamera instances
  *
- * This method shall not be called from a pipeline handler implementation
+ * This function shall not be called from a pipeline handler implementation
  * directly, as the base PipelineHandler implementation handles this on the
  * behalf of the specified implementation.
  *