[css-fonts-4] Add small-caps value to font-synthesis property

This patch also migrates this property from css-fonts-3 to css-fonts-4.

Discussion at https://lists.w3.org/Archives/Public/www-style/2015Jul/0463.html

Resolution recorded at:
https://lists.w3.org/Archives/Public/www-style/2015Aug/0109.html
"RESOLVED: support small-caps in font-synthesis in Fonts 4"

Author: litherum

css-fonts-4/Overview.bs

@@ -451,6 +451,33 @@ at least as dark as its predecessor. Only values between 1 - 999 are valid, and
 
 	TODO: Migrate this from level 3.
 
+<h3 id="font-size-adjust-prop">Controlling synthetic faces: the 'font-synthesis' property</h3>
+
+	<pre class="propdef">
+	Name: font-synthesis
+	Value: none | [ weight || style || small-caps ]
+	Initial: weight style small-caps
+	Applies to: all elements
+	Inherited: yes
+	Percentages: N/A
+	Computed value: as specified
+	Media: visual
+	Animatable: no
+	</pre>
+
+	This property controls whether user agents are allowed to synthesize bold or oblique font
+	faces when a font family lacks bold or italic faces.  If 'weight' is not specified, user agents
+	must not synthesize bold faces. If 'style' is not specified, user agents must not synthesize
+	italic faces. If 'small-caps' is not specified, user agents must not synthesize small-caps faces.
+	A value of 'none' disallows all synthetic faces.
+
+	<div class="example">
+	<p>The style rule below disables the use of synthetically obliqued Arabic:</p>
+	</p>
+	<pre>*:lang(ar) { font-synthesis: none; }
+	</pre>
+	</div>
+
 <h2 id="font-resources">Font Resources</h2>
 
 <h3 id="src-desc">Font reference: the 'src' descriptor</h3>
@@ -1412,13 +1439,6 @@ of predefined CSS generic font families to cover these type styles.</p>
 Issue: Add ''system-ui'' fingerprinting issue to Security and Privacy Considerations section
 per <a href="https://lists.w3.org/Archives/Public/www-style/2015Aug/0051.html">discussion</a>.
 
-<h2 id="font-synthesis-small-caps">New <code>small-caps</code> value for <code>font-synthesis</code></h2>
-
-<p>Since small capitals are often synthesized for fonts lacking smalls-caps glyphs,
-add a <code>small-caps</code> value to the <code>font-synthesis</code> property
-to allow small-caps synthesis to be disabled.
-(<a href="https://lists.w3.org/Archives/Public/www-style/2015Jul/0463.html">www-style discussion</a>)</p>
-
 <h2 id="emoji-fallback">Font fallback for emoji codepoints</h2>
 
 <p>Many codepoints supported by emoji fonts overlap with codepoints supported