Add documentation on query string and simple query string backends

[barseghyanartur]

For simple query string:

    query PostsQuery {
      allPostDocuments(
        simpleQueryString:"'trouble candidate' +Python"
      ) {
        edges {
          node {
            id
            title
            category
            content
            createdAt
            comments
          }
        }
      }
    }

For query string:

    query PostsQuery {
      allPostDocuments(
        queryString:"(trouble candidate) AND Python"
      ) {
        edges {
          node {
            id
            title
            category
            content
            createdAt
            comments
          }
        }
      }
    }