statvfs.3: Document missing 'f_flag' bit values

And reorganize information relating to which flags are in
POSIX.1.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/statvfs.3

@@ -78,16 +78,46 @@ Both used to be
 
 The field
 .I f_flag
-is a bit mask (of mount flags, see
-.BR mount (8)).
-Bits defined by POSIX are
+is a bit mask indicating various options that were employed
+when mounting this filesystem.
+It contains zero or more of the following flags:
+.\" XXX Keep this list in sync with statfs(2)
 .TP
-.B ST_RDONLY
-Read-only filesystem.
+.B ST_MANDLOCK
+Mandatory locking is permitted on the filesystem (see
+.BR fcntl (2)).
+.TP
+.B ST_NOATIME
+Do not update access times; see
+.BR mount (2).
+.TP
+.B ST_NODEV
+Disallow access to device special files on this filesystem.
+.TP
+.B ST_NODIRATIME
+Do not update directory access times; see
+.BR mount (2).
+.TP
+.B ST_NOEXEC
+Execution of programs is disallowed on this filesystem.
 .TP
 .B ST_NOSUID
-Set-user-ID/set-group-ID bits are ignored by
-.BR exec (3).
+The set-user-ID and set-group-ID bits are ignored by
+.BR exec (3)
+for executable files on this filesystem
+.TP
+.B ST_RDONLY
+This filesystem is mounted read-only.
+.TP
+.B ST_RELATIME
+Update atime relative to mtime/ctime; see
+.BR mount (2).
+.TP
+.B ST_SYNCHRONOUS
+Writes are synched to the filesystem immediately (see the description of
+.B O_SYNC
+in
+.BR open (2)).
 .LP
 It is unspecified whether all members of the returned struct
 have meaningful values on all filesystems.
@@ -165,6 +195,16 @@ and
 functions are thread-safe.
 .SH CONFORMING TO
 POSIX.1-2001.
+
+Only the
+.B ST_NOSUID
+and
+.B ST_RDONLY
+flags of the
+.I f_flag
+field are specified in POSIX.1.
+To obtain definitions of the remaining flags, one must define
+.BR _GNU_SOURCE .
 .SH NOTES
 The Linux kernel has system calls
 .BR statfs (2)