Port CLJ-2225: Improve docstrings of assert and *assert*

Co-authored-by: Alex Miller <alex.miller@cognitect.com>

Author: swannodette

src/main/cljs/cljs/core.cljs

@@ -69,7 +69,9 @@
   *out* nil)
 
 (def
-  ^{:dynamic true}
+  ^{:dynamic true
+    :doc "When set to logical false, 'assert' will omit assertion checks in
+ compiled code. Defaults to true."}
   *assert* true)
 
 (defonce

src/main/clojure/cljs/core.cljc

@@ -2479,8 +2479,11 @@
   (core/when *assert* x))
 
 (core/defmacro assert
-  "Evaluates expr and throws an exception if it does not evaluate to
-  logical true."
+  "Evaluates expression x and throws an exception with optional
+  message if x does not evaluate to logical true.
+
+  Assertion checks are omitted from compiled code if '*assert*' is
+  false."
   ([x]
      (core/when *assert*
        `(when-not ~x