fanotify.7, fanotify_mark.2: Clarify FAN_ONDIR in output mask

fanotify.7, fanotify_mark.2: Clarify FAN_ONDIR in output mask

FAN_ONDIR was an input only flag before introducing
FAN_REPORT_FID.  Since the introduction of FAN_REPORT_FID, it can
also be in output mask.

Move the text describing its role in the output mask to fanotify.7
where the other output mask bits are documented.

[mtk: commit message tidy-up]

Reviewed-by: Matthew Bobrowski <mbobrowski@mbobrowski.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>