docs(theme): correct the --rail-btn-inset comment — it reduces the duplication, it does not remove it

The prior wording claimed the two rules 'cannot drift apart', which is only true
for the documented composition. 0.6rem still appears twice; the fallback confines
a stale copy to the undocumented modifier-alone path. Records why hoisting the
var to :root would be worse rather than better.

ZB.MOM.WW.Theme/src/ZB.MOM.WW.Theme/wwwroot/css/layout.css

@@ -250,7 +250,18 @@
 
    calc() gets both right (measured 0px short for <a> and <button> alike). The
    inset is read from .rail-btn's own --rail-btn-inset rather than restated here,
-   so the two rules cannot drift apart if the rail inset ever changes.
+   so changing the rail inset there carries correctly into this width.
+
+   This REDUCES the duplication rather than removing it: 0.6rem still appears
+   twice — once as the declared value above, once as the fallback below — and
+   nothing keeps them in step. What changed is WHERE a stale copy can bite. The
+   documented `rail-btn rail-btn-block` composition resolves the var and follows
+   the real value; only the undocumented modifier-alone case can go stale. That
+   is the right place to leave it. The alternative — hoisting --rail-btn-inset to
+   a scope outliving the component (:root, .side-rail) — is worse: it makes a
+   component-private measurement look like a kit-wide token that apps may
+   reasonably override, and an app overriding it would silently resize a control
+   it never meant to touch.
 
    The 0.6rem fallback is NOT decoration. --rail-btn-inset is declared on
    .rail-btn, so it only resolves for the documented `rail-btn rail-btn-block`