diff --git a/CHANGELOG.md b/CHANGELOG.md
index c2901af5..badc72e6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 Version 137:
 
 * ConstBufferSequence mandates pointer equivalence
+* Add FieldsWriter constructor requirement
 
 --------------------------------------------------------------------------------
 
diff --git a/doc/qbk/07_concepts/FieldsWriter.qbk b/doc/qbk/07_concepts/FieldsWriter.qbk
index 55c2ad98..2c46adfd 100644
--- a/doc/qbk/07_concepts/FieldsWriter.qbk
+++ b/doc/qbk/07_concepts/FieldsWriter.qbk
@@ -58,6 +58,16 @@ In this table:
         response. The lifetime of `f` is guaranteed
         to end no earlier than after the `W` is destroyed.
     ]
+][
+    [`W{f}`]
+    []
+    [
+        The implementation calls this constructor to indicate
+        that the fields being serialized form part of a chunked
+        encoding final-chunk trailer. The lifetime of `f` is
+        guaranteed to end no earlier than after the `W` is
+        destroyed.
+    ]
 ][
     [`a.get()`]
     [`W::const_buffers_type`]