mm/doc: fix fault_flag_allow_retry_first kerneldoc

make htmldocs reports:

  include/linux/mm.h:496: warning: Function parameter or member 'flags' not described in 'fault_flag_allow_retry_first'

Add a description.

Link: https://lkml.kernel.org/r/20210322195022.2143603-1-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -485,6 +485,7 @@ extern pgprot_t protection_map[16];
 
 /**
  * fault_flag_allow_retry_first - check ALLOW_RETRY the first time
+ * @flags: Fault flags.
  *
  * This is mostly used for places where we want to try to avoid taking
  * the mmap_lock for too long a time when waiting for another condition