diff --git a/src/taoensso/nippy.clj b/src/taoensso/nippy.clj
index 626671f..f202f69 100644
--- a/src/taoensso/nippy.clj
+++ b/src/taoensso/nippy.clj
@@ -350,7 +350,28 @@
 
 ;; For back compatibility (nb Timbre's Carmine appender)
 (enc/defonce ^:dynamic ^:no-doc ^:deprecated *final-freeze-fallback* "DEPRECATED: prefer `*freeze-fallback`."       nil)
-(enc/defonce ^:dynamic                             *freeze-fallback* "(fn [data-output x])->freeze, nil => default" nil)
+(enc/defonce ^:dynamic                             *freeze-fallback*
+  "Controls Nippy's behaviour when trying to freeze an item for which Nippy
+  doesn't currently have a native freeze/thaw implementation.
+
+  Possible values:
+
+    1. `nil` (no freeze-fallback, default)
+       Tries the following in order:
+         - Freeze with Java's `Serializable` interface if possible
+         - Freeze with Clojure's reader                if possible
+         - Throw
+
+    2. `:write-unfreezable` keyword
+       Tries the following in order:
+         - Freeze with Java's `Serializable` interface if possible
+         - Freeze with Clojure's reader                if possible
+         - Freeze a {:nippy/unfreezable {:type _}} placeholder value
+
+    3. [Advanced] Custom (fn [^java.io.DataOutput out item]) that must
+       write exactly one value to the given `DataOutput` stream"
+
+  nil)
 
 (enc/defonce ^:dynamic *custom-readers* "{<hash-or-byte-id> (fn [data-input])->read}" nil)
 (enc/defonce ^:dynamic *auto-freeze-compressor*