bigquery: need additional documentation for tags

[septianhari-afk]

Is your feature request related to a problem? Please describe.

The documentation on https://pkg.go.dev/cloud.google.com/go/bigquery seems doesn't tells about how to use tags on query result

Describe the solution you'd like

I just want to add some line that we could use bigquery tags so the struct doesnt have to be exactly same with the query. E.g:

type Count struct {
    YearCount int `bigquery:"year"`
    NumCount  int `bigquery:"num"`
}

Describe alternatives you've considered

N/A

Additional context

N/A