docs(swingset): stop documenting icon overrides as SVG-only

`appearance.icons` clones whatever element it is given; nothing requires an
`svg`. The Icon tests override with a `span`.

Author: alexcarpenter

packages/swingset/src/stories/icon.mdx

@@ -51,7 +51,7 @@ Set `placement` when the icon sits beside text inside a container. It adds no st
 
 ### Overriding a glyph
 
-Pass `appearance.icons` to `MosaicProvider` to replace a glyph by name. The override is a plain element; Mosaic clones it with the resolved sizing `className` and `data-size`, keeping any class the element already had. The replacement only supplies its `viewBox` and paths.
+Pass `appearance.icons` to `MosaicProvider` to replace a glyph by name. The override is a plain element of any type, not just an `svg`; Mosaic clones it with the resolved sizing `className` and `data-size`, keeping any class the element already had. The replacement only supplies its own content.
 
 <Story
   name='Override'

packages/swingset/src/stories/icon.stories.tsx

@@ -87,9 +87,9 @@ export function Override() {
     <MosaicProvider
       appearance={{
         // Overrides are elements, not render functions: Mosaic injects its sizing className and
-        // `data-size` into the element via cloneElement, so the replacement only needs its viewBox
-        // + paths. Passing an element (vs a function) also lets overrides be supplied from a Server
-        // Component, since elements serialize across the RSC boundary.
+        // `data-size` into the element via cloneElement, so the replacement only supplies its own
+        // content and need not be an `svg`. Passing an element (vs a function) also lets overrides
+        // be supplied from a Server Component, since elements serialize across the RSC boundary.
         icons: {
           'chevron-right': (
             <svg