Add description for StylePropertyMapReadOnly.has() (#442)

Author: nainar

css-typed-om/Overview.bs

@@ -302,6 +302,17 @@ The <dfn method for=StylePropertyMap>append(DOMString <var>property</var>,
     4. Else, return `null`.
 </div>
 
+<div algorithm>
+    To <dfn>check if StylePropertyMap has a property</dfn>, run these steps:
+
+    1. Run the algorithm to [=get a value from a StylePropertyMap=] with property <var>property</var>,
+        1. If the algoritm returns a {{CSSStyleValue}} return true.
+
+        2. If the algorithm returns `null` return false.
+
+</div>
+
+
 <div algorithm>
     To <dfn>set a value on a StylePropertyMap</dfn>, run these steps: