Updated alt text

Author: radarhere

docs/deprecations.rst

@@ -257,7 +257,7 @@ offset of the text, while the new ``bbox`` methods distinguish this as a ``top``
 offset.
 
 .. image:: ./example/size_vs_bbox.png
-    :alt: Demonstration of size height vs bbox top and bottom
+    :alt: In bbox methods, top measures the vertical distance above the text, while bottom measures that plus the vertical distance of the text itself. In size methods, height also measures the vertical distance above the text plus the vertical distance of the text itself.
     :align: center
 
 FreeTypeFont.getmask2 fill parameter