doc: add HPE_UNEXPECTED_CONTENT_LENGTH error description

PR-URL: #34596
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>

doc/api/errors.md

@@ -2183,6 +2183,17 @@ malconfigured clients, if more than 8KB of HTTP header data is received then
 HTTP parsing will abort without a request or response object being created, and
 an `Error` with this code will be emitted.
 
+<a id="HPE_UNEXPECTED_CONTENT_LENGTH"></a>
+### `HPE_UNEXPECTED_CONTENT_LENGTH`
+
+Server is sending both a `Content-Length` header and `Transfer-Encoding: chunked`.
+
+`Transfer-Encoding: chunked` allows the server to maintain an HTTP persistent
+connection for dynamically generated content.
+In this case, the `Content-Length` HTTP header cannot be used.
+
+Use `Content-Length` or `Transfer-Encoding: chunked`.
+
 <a id="MODULE_NOT_FOUND"></a>
 ### `MODULE_NOT_FOUND`
 <!-- YAML