CLJS-1527: Doc for *print-length* copy of *print-dup*

Copy over docstring from Clojure.

Author: mfikes

src/main/cljs/cljs/core.cljs

@@ -104,10 +104,13 @@
 
 (def
   ^{:dynamic true
-    :doc "When set to logical true, objects will be printed in a way that preserves
-  their type when read in later.
-
-  Defaults to false."
+    :doc "*print-length* controls how many items of each collection the
+  printer will print. If it is bound to logical false, there is no
+  limit. Otherwise, it must be bound to an integer indicating the maximum
+  number of items of each collection to print. If a collection contains
+  more items, the printer will print items up to the limit followed by
+  '...' to represent the remaining items. The root binding is nil
+  indicating no limit."
     :jsdoc ["@type {null|number}"]}
   *print-length* nil)