fix: a size worked out from a bare diameter keeps its unit when the design's units change (#716) (#779)

A size typed as a bare diameter now keeps the design's length unit, so it no longer changes when the design's units change. Fixes #716.

docs/book/src/guide/parameters.md

@@ -25,7 +25,11 @@ in use can't be deleted (the error lists its users).
 The design's units (Document Settings in the browser: mm, cm, m, in or ft) set how a bare number
 typed into a feature is read. In an inch design `2` means 2 in. The Box, Cylinder, Sphere, Torus
 and Coil dialogs start with their sizes in the design's units, unit shown (`20 mm`, `20 in`).
-Changing the units never moves existing geometry.
+Changing the units never moves existing geometry. A size worked out from a bare number keeps
+the unit the number was given in: a sphere with a bare diameter of 20 has the radius
+`(20 mm) / 2` in a millimetre design. Beside a unit-less expression such as `2 * 5` or a
+unit-less parameter, which is read in the design's units, the number stays bare and is read
+with it.
 
 Parameters export to and import from CSV or JSON (`parameters.export`, `parameters.import`).
 **Configurations** (`config.*`) keep a table of rows, each with its own parameter values and