doc: opt db-ql documents

[yenche123]

const res = await db.collection("user").doc("644148fd1eeb2b524dba499e").get()
console.log(res.data)    // it might be null

Just to make the type I mentioned above more accurate

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[nightwhite]

[yenche123]

@nightwhite
In the screenshot you gave, there was a code block:

// 查询 user 集合中 name 字段等于 jack 的记录
await db.collection("user").where({
 name:"jack"
});

However, where returns a Query instead of Promise, so why did you use await?
It makes people confused.