/**
 * Respond to node deletion.
 *
 * This hook is invoked from node_delete_multiple() after the node has been
 * removed from the node table in the database, after the type-specific
 * hook_delete() has been invoked, and before field_attach_delete() is called.
 *
 * @param $node
 *   The node that is being deleted.
 *
 * @ingroup node_api_hooks
 * @ingroup hooks
 */
function hook_node_delete($node) {
  db_delete('mytable')
    ->condition('nid', $node->nid)
    ->execute();
}

The documentation for a hook definition follows the same guidelines as for regular functions (see above), except that the first line is a verb in the imperative form, e.g. "Respond to xyz". Also, a sample function body is provided, usually from a core module's implementation of the hook. It is also helpful to mention somewhere in the documentation where the hook is invoked.