Define rclone's remote where it is first used

The documents use "remote key" for something else, so "remote" as
rclone's noun is now defined on first use: a remote is a storage service
set up in rclone. Changed in `README.md` (the destination bullet),
`docs/REPOSTRUCTURE.md` (the opening) and `config.example.yml` (the
`storage_url` comment).

Model: fable-5-1

config.example.yml

@@ -278,8 +278,8 @@ exclude:
 # An s3:// URL also needs s3.access_key_id and s3.secret_access_key below
 # A file:// path must be absolute, and the directory must already exist:
 # vaultik never creates it
-# An rclone:// URL names a remote of rclone (https://rclone.org/); the remote
-# must be defined in rclone's own config (rclone config)
+# An rclone:// URL names an rclone remote (https://rclone.org/), a storage
+# service set up in rclone's own config (rclone config)
 storage_url: "rclone://myremote/path/to/backups"
 
 # S3-compatible storage configuration