Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (c *Collection) UpdateAll(selector interface{}, update interface{}) (info *ChangeInfo, err error) {
	lerr, err := c.writeQuery(&updateOp{c.FullName, selector, update, 2})
	if err == nil && lerr != nil {
		info = &ChangeInfo{Updated: lerr.N}
	}
	return info, err
}
func (c *Collection) UpdateAll(selector interface{}, update interface{}) (info *ChangeInfo, err error) {
	lerr, err := c.writeQuery(&updateOp{c.FullName, selector, update, 2})
	if err == nil && lerr != nil {
		info = &ChangeInfo{Updated: lerr.N}
	}
	return info, err
}
// UpdateAll finds all documents matching the provided selector document
// and modifies them according to the update document.
// If the session is in safe mode (see SetSafe) details of the executed
// operation are returned in info or an error of type *LastError when
// some problem is detected. It is not an error for the update to not be
// applied on any documents because the selector doesn't match.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Updating
//     http://www.mongodb.org/display/DOCS/Atomic+Operations
//