take suggestions from PR comment https://github.com/napari/docs/pull/…

…367/files\#r1517021908

docs/plugins/building_a_plugin/_layer_data_guide.md

@@ -8,8 +8,9 @@ up often in plugins and is explained here.
 
 Note that when writing your own plugin, type annotations are optional, 
 except in the case of [`magicgui` function widgets](magicgui).
-Here we sometimes provide a type annotation with a more informative name, 
-indicating its value's _functional role_ in a plugin, in addition to its possible types.
+For several types related to `LayerData` tuples, Napari defines an alias 
+which better indicates a value's _functional role_ in a plugin. 
+We describe these below.
 
 ### Informal description