libcamera: Document missing parameters and return values

Several functions are missing documentation for some of them parameters
and/or for their return value. Fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/device_enumerator.cpp

@@ -124,6 +124,9 @@ bool DeviceMatch::match(const MediaDevice *device) const
  * implementations may be needed. This function creates the best enumerator for
  * the operating system based on the available resources. Not all different
  * enumerator types are guaranteed to support all features.
+ *
+ * \return A pointer to the newly created device enumerator on success, or
+ * nullptr if an error occurs
  */
 DeviceEnumerator *DeviceEnumerator::create()
 {