grafana-tidb-dashboard.md: Store Query Feedback QPS description accuracy unclear ("union query")

What is changed, added or deleted? (Required)

The "Store Query Feedback QPS" panel description reads:

- Store Query Feedback QPS: the number of operations per second to store the feedback information of the union query, which is performed in TiDB memory

While translating this into Japanese, I tried to verify the meaning of "union query" here (does it refer to SQL's UNION operator specifically, or something else?) and could not confirm it against available sources:

1. The actual Grafana dashboard JSON (pingcap/monitoring, e.g. monitor/v4.0.0-beta.2/dashboards/tidb.json, panel id 170) has a much simpler built-in description: "TiDB store quering feedback counts" — no mention of "union" at all. Its query is sum(rate(tidb_statistics_store_query_feedback_total[1m])) by (type).
2. The Chinese source (docs-cn/grafana-tidb-dashboard.md) uses "合并查询" (roughly "merged/combined query"), which is a different concept from SQL UNION.
3. Searching TiDB source for the underlying StoreQueryFeedbackCounter metric (session.StoreQueryFeedback) shows the type label used is just ok/error (success/failure of storing feedback), with no reference to "union" or "merge" query semantics.

Given (1) and (3), it's unclear whether "union query" is accurate, and if so, what it's meant to convey (this statistics-feedback mechanism may be legacy/deprecated, since the underlying metric/counter doesn't appear to exist in current pingcap/tidb master). Could someone with more context on the statistics feedback mechanism confirm whether "union query" is accurate here, and if not, suggest a corrected description?

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)

What is the related PR or file link(s)?

• Other reference link(s): found while reviewing the Japanese translation of this file (PR #23763)

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

Start with grafana-tidb-dashboard.md and compare its Store Query Feedback QPS description with docs-cn/grafana-tidb-dashboard.md and pingcap/monitoring's tidb.json panel 170. Check the referenced StoreQueryFeedbackCounter metric and query labels; done means confirming the meaning of “union query” and updating the description if it is inaccurate.