expand a little the explanation about discard

describe behaviour of combination of log+async (i.e. same as just log)

sbin/mount/mount.8

@@ -1,4 +1,4 @@
-.\"	$NetBSD: mount.8,v 1.82 2016/11/10 22:28:00 jdolecek Exp $
+.\"	$NetBSD: mount.8,v 1.83 2017/03/22 21:14:11 jdolecek Exp $
 .\"
 .\" Copyright (c) 1980, 1989, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -29,7 +29,7 @@
 .\"
 .\"     @(#)mount.8	8.8 (Berkeley) 6/16/94
 .\"
-.Dd November 10, 2016
+.Dd March 22, 2017
 .Dt MOUNT 8
 .Os
 .Sh NAME
@@ -198,10 +198,15 @@ mode.
 Use DISCARD/TRIM commands if disk and driver support it.
 .Pp
 .Em EXPERIMENTAL
-- negatively influences filesystem performance,
+- negatively influences filesystem performance by increasing fragmentation,
 causes free block map inconsistency on unclean shutdown,
 and is incompatible with
 .Cm log .
+If
+.Cm log
+option is also used,
+.Cm discard
+is automatically disabled.
 .It Cm extattr
 Enable extended attributes, if the filesystem supports them and
 does not enable them by default.
@@ -281,10 +286,6 @@ Mount the file system with
 meta-data journaling, also known simply as logging.
 It provides rapid metadata updates and eliminates the need to check
 file system consistency after a system outage.
-A file system mounted with
-.Cm log
-can not be mounted with
-.Cm async .
 It requires the
 .Dv WAPBL
 option to be enabled in the running kernel.
@@ -301,6 +302,20 @@ use the
 .Fl c
 option of
 .Xr fsck_ffs 8 .
+.Pp
+.Sp
+.Cm log
+cannot be used together with
+.Cm discard .
+.Pp
+A file system mounted with
+.Cm log
+can be mounted also with
+.Cm async ,
+but such filesystem behaves the same as if
+.Cm async
+was not specified - meta-data writes use the log, hence its integrity
+is still guaranteed.
 .It Cm symperm
 Recognize permission of symbolic link when reading or traversing link.
 .It Cm sync