Modifies an existing DB subnet group. DB subnet groups must contain at least one subnet in at least two AZs in the region.
/// Subnets in the DB subnet group should cover at least two Availability Zones unless
/// there is only one Availability Zone.
///
/// DBSubnetGroupName does not refer to an existing DB subnet group.
///
/// Request would result in user exceeding the allowed number of subnets in a DB subnet
/// groups.
///
/// The requested subnet is invalid, or multiple subnets were requested that are not all
/// in a common VPC.
///
/// The DB subnet is already in use in the Availability Zone.
///
