doc: model schema document fix and wording about the model price parameter (#2944)

api/core/model_runtime/docs/en_US/schema.md

@@ -147,7 +147,7 @@
 
 - `input` (float) Input price, i.e., Prompt price
 - `output` (float) Output price, i.e., returned content price
-- `unit` (float) Pricing unit, e.g., per 100K price is `0.000001`
+- `unit` (float) Pricing unit, e.g., if the price is meausred in 1M tokens, the corresponding token amount for the unit price is `0.000001`.
 - `currency` (string) Currency unit
 
 ### ProviderCredentialSchema

api/core/model_runtime/docs/zh_Hans/schema.md

@@ -149,7 +149,7 @@
 
 - `input` (float) 输入单价，即 Prompt 单价
 - `output` (float) 输出单价，即返回内容单价
-- `unit` (float) 价格单位，如：每 100K 的单价为 `0.000001`
+- `unit` (float) 价格单位，如以 1M tokens 计价，则单价对应的单位 token 数为 `0.000001`
 - `currency` (string) 货币单位
 
 ### ProviderCredentialSchema