open(2): fix the description of O_FSYNC

The man page claims that with O_FSYNC (aka O_SYNC) the kernel will not cache
written data. However, that's not true. Nor does POSIX require it.
Perhaps it was true when that section of the man page was written in r69336
(I haven't checked). But it's not true now.  Now the effect is simply that
writes are sent to disk immediately and synchronously, but they're still
cached.

See also: https://pubs.opengroup.org/onlinepubs/9699919799/
See also: ffs_write in sys/ufs/ffs/ffs_vnops.c

Reviewed by:	cem
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D20641

lib/libc/sys/open.2

@@ -28,7 +28,7 @@
 .\"     @(#)open.2	8.2 (Berkeley) 11/16/93
 .\" $FreeBSD$
 .\"
-.Dd November 11, 2018
+.Dd June 14, 2019
 .Dt OPEN 2
 .Os
 .Sh NAME
@@ -224,10 +224,7 @@ The descriptor remains in non-blocking mode for subsequent operations.
 If
 .Dv O_FSYNC
 is used in the mask, all writes will
-immediately be written to disk,
-the kernel will not cache written data
-and all writes on the descriptor will not return until
-the data to be written completes.
+immediately and synchronously be written to disk.
 .Pp
 .Dv O_SYNC
 is a synonym for