microhttpd.h: deprecate unused MHD_RESPONSE_HEADER_KIND value

doc/libmicrohttpd.texi

@@ -1047,9 +1047,6 @@ The @code{MHD_ValueKind} specifies the source of the key-value pairs in
 the HTTP protocol.
 
 @table @code
-@item MHD_RESPONSE_HEADER_KIND
-Response header.
-
 @item MHD_HEADER_KIND
 HTTP header.
 

src/include/microhttpd.h

@@ -1526,11 +1526,15 @@ enum MHD_ValueKind
 
   /**
    * Response header
+   * @deprecated
    */
   MHD_RESPONSE_HEADER_KIND = 0,
+#define MHD_RESPONSE_HEADER_KIND \
+  _MHD_DEPR_IN_MACRO("Value MHD_RESPONSE_HEADER_KIND is deprecated and not used") \
+  MHD_RESPONSE_HEADER_KIND
 
   /**
-   * HTTP header.
+   * HTTP header (request/response).
    */
   MHD_HEADER_KIND = 1,