[css-shapes-2] Editorial: Add clarification about relationship with '…

…path' (#11743)

* [css-shapes-2] Editorial: Add clarification about relationship with 'path'

* Update css-shapes-2/Overview.bs

---------

Co-authored-by: Tab Atkins Jr. <[email protected]>

css-shapes-2/Overview.bs

@@ -147,13 +147,17 @@ The ''shape()'' Function</h4>
 	and inherits a number of limitations from SVG,
 	such as implicitly only allowing the ''px'' unit.
 
-	The ''shape()'' function uses a set of commands equivalent to the ones used by ''path()'',
+	The ''shape()'' function uses a set of commands roughly equivalent to the ones used by ''path()'',
 	but does so with more standard CSS syntax,
 	and allows the full range of CSS functionality,
 	such as additional units and math functions.
 	The commands used by ''shape()'' are dynamically turned into path segments when it is used for rendering,
 	e.g., when computing the rendered 'clip-path'.
 
+	In that sense, ''shape()'' is a superset of ''path()''. A ''path()'' can be easily converted to a ''shape()'',
+	but to convert a ''shape()'' back to a ''path()'' or to SVG requires information about the CSS environment (e.g.
+	current values of CSS custom properties, current font size for ''em'' units, etc).
+
 	<pre class=prod>
 		<dfn>shape()</dfn> = shape( <<'fill-rule'>>? from <<position>>, <<shape-command>>#)
 	</pre>