make shorthands more intuitive

docs/shorthands.rst

@@ -5,17 +5,20 @@ For the arguments ``x``, ``y``, ``width``, and ``height``, a few convenient shor
 
 * ``middle`` for ``x`` and ``width`` refer to the deck's width / 2
 * ``middle`` for ``y`` and ``height`` refer to the deck's height / 2
+* The word ``center`` behaves the same way
 * ``deck`` refers to the deck's width for ``x`` and ``width``
 * ``deck`` refers to the deck's height for ``y`` and ``height``
-* You can offset from the middle by using +, -, and /, e.g. ``middle + 1in``
-* You can offset from the width or height using, e.g. ``width - 1in`` or ``height - 2mm``
-* Works with the ``cell`` unit as well, e.g. `middle + 1 cell`. See :doc:`units`.
+* You can offset from the middle by using + or - operators, e.g. ``middle + 1in``
+* You can offset from the deck width or height using the + or - operators, e.g. ``deck - 1in`` or ``deck - 2mm``
+* You can offset from the deck width or height using, e.g. ``deck / 3``
+* Works with all unit conversion too, e.g. `middle + 1 cell`. See :doc:`units`.
 
 These are all passed as strings. So you will need to quote them in Ruby, or just plain in your layout YAML.
 
 Note that the following are NOT supported:
 
 * The `+=` operator when using `extends` in a layout file
+* Complicated formulas. We're not evaluating this as code, we're looking for these specific patterns and applying them. Anything more complicated you'll have to handle with Ruby code.
 
 Samples
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