[fstab] Add some discussion about *discard* option

- drop the *discard* from filesystems-on-SSD in the standard example
  configuration.
- keep the table **with** *discard* around for referece and explanation.

Remember that the example configurations are intended as **examples**,
to document available settings, and do not reflect a sensible
production configuration.

FIXES #1395

src/modules/fstab/fstab.conf

@@ -20,12 +20,25 @@ efiMountOptions: umask=0077
 # If a filesystem is on an SSD, add the following options. If a specific
 # filesystem is listed here, use those options, otherwise no additional
 # options are set (i.e. there is no *default* like in *mountOptions*).
+#
+# This example configuration applies the *discard* option to most
+# common filesystems on an SSD. This may not be the right option
+# for your distribution. If you use a systemd timer to trim the
+# SSD, it may interfere with the *discard* option. Opinions vary
+# as to whether *discard* is worth the effort -- it depends on
+# the usage pattern of the disk as well.
+#
+# ssdExtraMountOptions:
+#     ext4: discard
+#     jfs: discard
+#     xfs: discard
+#     swap: discard
+#     btrfs: discard,compress=lzo
+#
+# The standard configuration applies only lzo compression to btrfs
+# and does nothing for other filesystems.
 ssdExtraMountOptions:
-    ext4: discard
+    btrfs: compress=lzo
-    jfs: discard
-    xfs: discard
-    swap: discard
-    btrfs: discard,compress=lzo
 
 # Additional options added to each line in /etc/crypttab
 crypttabOptions: luks