code: 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
  }
docs: // 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
  //