genalloc: Fix an incorrect kerneldoc comment

The kerneldoc comment for the genpool_algo_t typedef was incomplete and
incorrectly formatted, leading to a raft of warnings during the docs build.
Fix it appropriately.
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

include/linux/genalloc.h

@@ -38,12 +38,13 @@ struct device_node;
 struct gen_pool;
 
 /**
- * Allocation callback function type definition
+ * typedef genpool_algo_t: Allocation callback function type definition
  * @map: Pointer to bitmap
  * @size: The bitmap size in bits
  * @start: The bitnumber to start searching at
  * @nr: The number of zeroed bits we're looking for
- * @data: optional additional data used by @genpool_algo_t
+ * @data: optional additional data used by the callback
+ * @pool: the pool being allocated from
  */
 typedef unsigned long (*genpool_algo_t)(unsigned long *map,
 			unsigned long size,