[css-fonts-4] Add generic(nastaliq), w3c#4397

Author: svgeesus

css-fonts-4/Overview.bs

@@ -647,6 +647,20 @@ Generic font families</h4>
 			but if it does, that font will be in Kai style.
 		</dd>
 
+		<dt id="generic(nastaliq)-def"><dfn>generic(nastaliq)</dfn>
+		<dd>
+			This font family is the standard way of writing Urdu and Kashmiri, 
+			and is also often a preferred style for Persian and other language text, 
+			especially in literary genres such as poetry. 
+			Key features include a sloping baseline for joined letters, 
+			and overall complex shaping and positioning for base letters and diacritics alike.
+			There are also distinctive shapes for many glyphs and ligatures. 
+			It is important not to fall back to a naskh style 
+			for languages such as Urdu and Kashmiri.
+
+			Note: ''generic(nastaliq)'' might not map to any locally installed font,
+			but if it does, that font will be in Nastaliq style.
+
 	    <dt id="ui-serif-def"><dfn>ui-serif</dfn>
 	    <dd>
 	        This font family is used for the serif variant of the system's