include/linux/mm: fix release_pages_arg kernel doc comment

Commit 449c7967 ("mm: teach release_pages() to take an array of
encoded page pointers too") added the kernel doc comment for
release_pages() on top of 'union release_pages_arg', so making 'make
htmldocs' complains as below:

    ./include/linux/mm.h:1268: warning: cannot understand function prototype: 'typedef union '

The kernel doc comment for the function is already on top of the
function's definition in mm/swap.c, and the new comment is actually not
for the function but indeed release_pages_arg.  Fixing the comment to
reflect the intent would be one option.  But, kernel doc cannot parse
the union as below due to the attribute.

    ./include/linux/mm.h:1272: error: Cannot parse struct or union!

Modify the comment to reflect the intent but do not mark it as a kernel
doc comment.

Link: https://lkml.kernel.org/r/20230106203331.127532-1-sj@kernel.org
Fixes: 449c7967 ("mm: teach release_pages() to take an array of encoded page pointers too")
Signed-off-by: SeongJae Park <sj@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -1270,10 +1270,10 @@ static inline void folio_put_refs(struct folio *folio, int refs)
 		__folio_put(folio);
 }
 
-/**
- * release_pages - release an array of pages or folios
+/*
+ * union release_pages_arg - an array of pages or folios
  *
- * This just releases a simple array of multiple pages, and
+ * release_pages() releases a simple array of multiple pages, and
  * accepts various different forms of said page array: either
  * a regular old boring array of pages, an array of folios, or
  * an array of encoded page pointers.