Issue #2290267 by jhodgdon: Image-related plugin classes need more docs links.

.../lib/Drupal/Core/ImageToolkit/Annotation/ImageToolkit.php

...al/Core/ImageToolkit/Annotation/ImageToolkitOperation.php

core/lib/Drupal/Core/ImageToolkit/ImageToolkitBase.php

core/lib/Drupal/Core/ImageToolkit/ImageToolkitInterface.php

core/lib/Drupal/Core/ImageToolkit/ImageToolkitManager.php

...ib/Drupal/Core/ImageToolkit/ImageToolkitOperationBase.php

...upal/Core/ImageToolkit/ImageToolkitOperationInterface.php

...Drupal/Core/ImageToolkit/ImageToolkitOperationManager.php

core/modules/image/src/Annotation/ImageEffect.php

core/modules/image/src/ConfigurableImageEffectBase.php

core/modules/image/src/ConfigurableImageEffectInterface.php

core/modules/image/src/ImageEffectBase.php

core/modules/image/src/ImageEffectInterface.php

core/modules/image/src/ImageEffectManager.php