[docs] Add alt text to image in Parameters-Tuning.rst (#4035)

* [docs] Add alt text to image in Parameters-Tuning.rst

Add alt text to Leaf-wise growth image, as part of #4028

* Update docs/Parameters-Tuning.rst
Co-authored-by: James Lamb <jaylamb20@gmail.com>
Co-authored-by: James Lamb <jaylamb20@gmail.com>

docs/Parameters-Tuning.rst

@@ -70,6 +70,7 @@ LightGBM adds nodes to trees based on the gain from adding that node, regardless
 
 .. image:: ./_static/images/leaf-wise.png
    :align: center
+   :alt: Three consecutive images of decision trees, where each shows the tree with an additional two leaf nodes added. Shows that leaf-wise growth can result in trees that have some branches which are longer than others.
 
 Because of this growth strategy, it isn't straightforward to use ``max_depth`` alone to limit the complexity of trees. The ``num_leaves`` parameter sets the maximum number of nodes per tree. Decrease ``num_leaves`` to reduce training time.