docs: add short descriptions for main binaries.

docs/proxmox-backup-client/description.rst

@@ -1,4 +1,4 @@
-This is just a test.
+This tool implements a backup server client, i.e. it can connect to a
-
+backup servers to issue management commands and to create or restore
-.. NOTE:: No further info.
+backups.
 

docs/proxmox-backup-manager/description.rst

@@ -1,4 +1,2 @@
-This is just a test.
+This tool exposes the whole backup server management API on the
-
+command line.
-.. NOTE:: No further info.
-

docs/proxmox-tape/description.rst

@@ -1,4 +1 @@
-This is just a test.
+This tool can configure and manage tape backups.
-
-.. NOTE:: No further info.
-