docs(bigquery): add numeric and bignumeric to RowIterator docs (#6560)

Document field type conversions for `NUMERIC` and `BIGNUMERIC` types when using the `RowIterator` for reading data from BigQuery. 

Solves issue #6492

Author: alvarowolfx

bigquery/integration_test.go

@@ -1190,7 +1190,7 @@ func TestIntegration_InsertAndReadNullable(t *testing.T) {
 	ctm := civil.Time{Hour: 15, Minute: 4, Second: 5, Nanosecond: 6000}
 	cdt := civil.DateTime{Date: testDate, Time: ctm}
 	rat := big.NewRat(33, 100)
-	rat2 := big.NewRat(66, 100)
+	rat2 := big.NewRat(66, 10e10)
 	geo := "POINT(-122.198939 47.669865)"
 
 	// Nil fields in the struct.

bigquery/iterator.go

@@ -114,6 +114,12 @@ type pageFetcher func(ctx context.Context, _ *rowSource, _ Schema, startIndex ui
 //	DATE        civil.Date
 //	TIME        civil.Time
 //	DATETIME    civil.DateTime
+//	NUMERIC     *big.Rat
+//	BIGNUMERIC  *big.Rat
+//
+// The big.Rat type supports numbers of arbitrary size and precision.
+// See https://cloud.google.com/bigquery/docs/reference/standard-sql/data-types#numeric-type
+// for more on NUMERIC.
 //
 // A repeated field corresponds to a slice or array of the element type. A STRUCT
 // type (RECORD or nested schema) corresponds to a nested struct or struct pointer.