chore(pdk): doc a known issue of get_headers() (#12006)

Adressing KAG-2602, #11546

Co-authored-by: Datong Sun <[email protected]>

kong/pdk/service/response.lua

@@ -198,6 +198,8 @@ local function new(pdk, major_version)
   --   kong.log.inspect(headers.x_another[1])    -- "foo bar"
   --   kong.log.inspect(headers["X-Another"][2]) -- "baz"
   -- end
+  -- Note that this function returns a proxy table
+  -- which cannot be iterated with `pairs` or used as operand of `#`.
   function response.get_headers(max_headers)
     check_phase(header_body_log)