fanotify: Document FAN_REPORT_PIDFD Feature

fanotify: Document FAN_REPORT_PIDFD Feature

Update the fanotify API documentation to include details on the new
FAN_REPORT_PIDFD feature. This patch also includes a generic section
describing the concept of information records which are supported by
the fanotify API.

Signed-off-by: Matthew Bobrowski <repnop@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
[alx: wsfix, ffix]
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>