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/ipa/raspberrypi/raspberrypi.cpp

@@ -741,7 +741,7 @@ void IPARPi::queueRequest(const ControlList &controls)
 			}
 
 			/*
-			 * The SetEv() method takes in a direct exposure multiplier.
+			 * The SetEv() function takes in a direct exposure multiplier.
 			 * So convert to 2^EV
 			 */
 			double ev = pow(2.0, ctrl.second.get<float>());