mm/damon: add documentation for Enum value

Fix the warning - "Enum value 'NR_DAMON_OPS' not described in enum
'damon_ops_id'" generated by the command "make pdfdocs"

Link: https://lkml.kernel.org/r/20220508073316.141401-1-gautammenghani201@gmail.comSigned-off-by: Gautam Menghani <gautammenghani201@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/damon.h

@@ -264,6 +264,7 @@ struct damos {
  * @DAMON_OPS_FVADDR:	Monitoring operations for only fixed ranges of virtual
  *			address spaces
  * @DAMON_OPS_PADDR:	Monitoring operations for the physical address space
+ * @NR_DAMON_OPS:	Number of monitoring operations implementations
  */
 enum damon_ops_id {
 	DAMON_OPS_VADDR,