[codex] clarify tree emptyCircle itemStyle docs

[Justin-ZS]

Summary

• clarify that series.tree uses 'emptyCircle' by default and that its inner fill is reserved for collapsed-state indication
• document that, with the default hollow symbol, itemStyle.color affects the outer ring while borderColor / borderWidth do not behave like they do on solid symbols
• add the same clarification to both English and Chinese docs

Why

The current series-tree.itemStyle docs say that itemStyle.color is the node fill color. That is misleading for the default 'emptyCircle' symbol, and it makes issue reports like apache/echarts#21582 look like runtime bugs when the actual mismatch is in documentation.

Validation

• npm run build
  • doc generation completed successfully
  • the build also reported a local asset-copy warning for missing public/js/doc-bundle.js, which is unrelated to these markdown changes