Add equivalent SQL examples for admin API requests

[JamesChenX]

No description provided.

[JamesChenX]

Movitation

All programmers should be familiar with the basic SQL statements, so it will be easier for users to understand the behavior of the new admin API after #1461.
e.g.:

POST /admins/query
{
  "skip": 20,
  "limit": 10,
  "projection": {
    "*": true
  },
  "filter": {
    "id": [1, 2, 3],
    "role": 1
  }
}

is equivalent to

select * from admins where id in (1, 2, 3) and role=1 limit 10 offset 20

Note that turms servers do NOT just send the SQL to MongoDB, but will run business logic before finding data in MongoDB. The SQL is just an example.