CLJS-1550: Enhance docstring for extend-type wrt type-sym

Author: mfikes

src/main/clojure/cljs/core.cljc

@@ -1472,8 +1472,22 @@
 
 (core/defmacro extend-type
   "Extend a type to a series of protocols. Useful when you are
-   supplying the definitions explicitly inline. Propagates the
-   type as a type hint on the first argument of all fns.
+  supplying the definitions explicitly inline. Propagates the
+  type as a type hint on the first argument of all fns.
+
+  type-sym may be
+
+   * default, meaning the definitions will apply for any value,
+     unless an extend-type exists for one of the more specific
+     cases below.
+   * nil, meaning the definitions will apply for the nil value.
+   * any of object, boolean, number, string, array, or function,
+     indicating the definitions will apply for values of the
+     associated base JavaScript types. Note that, for example,
+     string should be used instead of js/String.
+   * a JavaScript type not covered by the previous list, such
+     as js/RegExp.
+   * a type defined by deftype or defrecord.
 
   (extend-type MyType
     ICounted