Resolve the remote output path on the server

The output path was made absolute with os.path.abspath and '~' was
expanded to the local home, so relative paths and '~' pointed to the
wrong folder on the server. Resolve the path on the server instead,
where '~' and relative paths refer to the remote user's home, and
document it.

man/simple_backup.1

@@ -146,6 +146,14 @@ Copy it to the default location ($HOME/.config/simple_backup) and edit it as nee
 It is possible to choose a directory on a remote server as destination for the backup. The files
 are copied by rsync through SSH. Server hostname and username must be specified, either in the
 configuration file, or on the command line (\(aq\-\-ssh\-host\(aq and \(aq\-\-ssh\-user\(aq options).
+.P
+The output directory is a path on the server. A leading \(aq~\(aq and relative paths refer to the
+home directory of the user on the server. On the command line, quote \(aq~\(aq so that the local
+shell doesn\(aqt expand it, for example:
+.P
+.EX
+    simple_backup \-\-ssh\-host server \-\-ssh\-user user \-o \(aq~/backups\(aq
+.EE
 .SS AUTHENTICATION
 For authentication, it is possible to use SSH key or password.
 .P