docs: clarify ext-plugin-post-resp limitations

[moying2026]

Summary

Clarify that ext-plugin-post-resp uses lua-resty-http to send HTTP requests to upstreams, so it only works with HTTP upstreams and the HTTP response flow.

Changes

• Documented that ext-plugin-post-resp should not be used with gRPC transcoding or non-HTTP upstream response flows, such as grpc-transcode.
• Updated both English and Chinese plugin documentation with the same limitation.
• Added PR tracking details for this issue and the existing open PR fix: correct loggly syslog emergency severity #13390.

Testing

• Reviewed git diff for the documentation-only changes.
• Ran git diff --check.
• Ran grep -RInE 'grpc-transcode|gRPC|HTTP upstream|HTTP 上游|gRPC 转码' docs/en/latest/plugins/ext-plugin-post-resp.md docs/zh/latest/plugins/ext-plugin-post-resp.md PR_TRACKING.md to confirm the limitation is present in both languages.\n- Full documentation lint was not run because this is a minimal documentation-only change and no dedicated docs lint environment was identified during the fix.\n\nFixes help request: plugins question that cant use grpc-transcode with ext-plugin-post-resp #10787