Docs: Comments from @jix

- Unswap shift/shiftx
- Add brief overview to cell lib
- Clarify $div cell B input
- Clarify unary operators
- What is $modfloor

docs/source/cell/properties.rst

@@ -15,6 +15,7 @@ Cell properties
 .. cell:defprop:: x-output
 
    These cells can produce 'x' output even if all inputs are defined.  For
-   example, a `$div` cell with ``B=0`` has undefined output.
+   example, a `$div` cell with divisor (``B``) equal to zero has undefined
+   output.
 
 Refer to the :ref:`propindex` for the list of cells with a given property.