lua: mention that body should be consumed before fetching trailers (e…

…nvoyproxy#36553)

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Commit Message: lua: mention that body should be consumed before
fetching trailers
Additional Description:
Risk Level: Zero
Testing: N/A
Docs Changes: lua_filter.rst
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
Fixes envoyproxy#36507
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional [API
Considerations](https://github.com/envoyproxy/envoy/blob/main/api/review_checklist.md):]

Signed-off-by: spacewander <spacewanderlzx@gmail.com>
Signed-off-by: Gustavo <grnmeira@gmail.com>

docs/root/configuration/http/http_filters/lua_filter.rst

@@ -365,8 +365,10 @@ trailers()
 
   local trailers = handle:trailers()
 
-Returns the stream's trailers. May return nil if there are no trailers. The trailers may be
-modified before they are sent to the next filter.
+Returns the stream's trailers. Before calling this method, the caller should call ``body()`` or
+``bodyChunks()`` to consume the body, otherwise the trailers will not be available.
+May return nil if there are no trailers. The trailers may be modified before they are sent
+to the next filter.
 
 Returns a :ref:`header object <config_http_filters_lua_header_wrapper>`.