docs(config): the plan display name is the frontend's, not /api/plans (#371)

`Plan.name` claimed "为空时 /api/plans 按 type 推导" — the opposite of what the
endpoint does. `plans()` returns the config value verbatim (empty string when
unset) and there is a gate pinning that: a backend-invented label would surface
as Chinese text in the `en` UI, because response *data* fields are rendered
as-is (`I18n.mapErr` only translates the `error` field). The `type` → label
mapping lives in the frontend's `planLabel()`, which reads the language pack.

The comment outlived the behaviour it describes: the derivation was removed in
the same change that rewrote `plans()`'s own doc comment, which this struct
field never received. Point at the real owner instead of restating the history
that `gateway.rs` already carries.

Comment only — no behaviour, no signature, no config-shape change.

Author: argszero

src/config.rs

@@ -361,7 +361,9 @@ pub struct Provider {
 pub struct Plan {
     pub id: String,
     pub provider: String,
-    /// 显示名（可选；为空时 /api/plans 按 type 推导）
+    /// 显示名（可选）：`config.example.toml` 的 `[[plans]]` 一律不写它 ⇒ `/api/plans` 原样回传
+    /// 空串（语言中性标记，后端**不自造**）；显示文案由前端 `planLabel()` 按 `type` 取语言包键
+    ///（机制与缘由见 `src/gateway.rs` 的 `plans()`）。
     #[serde(default)]
     pub name: String,
     /// paygo | token | coding