btrfs-progs: docs: enhance subvol show documentation and help

There's a suggestion in #158 to improve the documenatation of the subvol
show command so it's more obvious what kind of information it provides.

I've updated the docs according to that and added an example that should
make the request for creation time at least greppable, the overall
description mentions 'times' so this should be enough as a pointer.

Pull-request: #158
Suggested-by: Robert Pollak <robert.pollak@posteo.net>
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-subvolume.asciidoc

@@ -171,21 +171,33 @@ path.
 The id can be obtained from *btrfs subvolume list*, *btrfs subvolume show* or
 *btrfs inspect-internal rootid*.
 
-*show* [options] <path>|<mnt>::
+*show* [options] <path>::
-Show information of a given subvolume in the <path>.
+Show more information about subvolume <path> regarding UUIDs, times,
+generations, flags and related snapshots.
++
+----------
+/mnt/btrfs/subvolume
+        Name:                   subvolume
+        UUID:                   5e076a14-4e42-254d-ac8e-55bebea982d1
+        Parent UUID:            -
+        Received UUID:          -
+        Creation time:          2018-01-01 12:34:56 +0000
+        Subvolume ID:           79
+        Generation:             2844
+        Gen at creation:        2844
+        Parent ID:              5
+        Top level ID:           5
+        Flags:                  -
+        Snapshot(s):
+----------
 +
 `Options`
 +
 -r|--rootid::::
 rootid of the subvolume.
--u|--uuid:::
+-u|--uuid::::
 UUID of the subvolume.
 
-+
-If no option is specified, subvolume information of <path> is shown,
-otherwise the subvolume information of rootid or UUID in the filesystem
-is shown.
-
 *snapshot* [-r|-i <qgroupid>] <source> <dest>|[<dest>/]<name>::
 Create a snapshot of the subvolume <source> with the
 name <name> in the <dest> directory.

cmds-subvolume.c

@@ -912,13 +912,13 @@ static int cmd_subvol_find_new(int argc, char **argv)
 }
 
 static const char * const cmd_subvol_show_usage[] = {
-	"btrfs subvolume show [options] <subvol-path>|<mnt>",
+	"btrfs subvolume show [options] <path>",
-	"Show more information about the subvolume",
+	"Show more information about the subvolume (UUIDs, generations, times, snapshots)",
 	"-r|--rootid   rootid of the subvolume",
 	"-u|--uuid     uuid of the subvolume",
 	"",
-	"If no option is specified, <subvol-path> will be shown, otherwise",
+	"If no option is specified, subvolume at <path> will be shown, otherwise",
-	"the rootid or uuid are resolved relative to the <mnt> path.",
+	"the rootid or uuid are resolved relative to the <path>.",
 	NULL
 };