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

README.md

@@ -36,7 +36,8 @@ It was written to meet these requirements together:
 - Small files are packed together into large objects. This keeps the number of
   S3 requests low.
 - File contents, names, paths and metadata are encrypted at the destination.
-- Three kinds of destination: S3, a local directory and any rclone remote.
+- Three kinds of destination: S3, a local directory and any rclone remote. A
+  remote is a storage service set up in rclone.
 - Regular files, symlinks and directories are backed up, with their permissions,
   owners and modification times.
 - Pure Go, with no C code. It builds for Linux and macOS on `amd64` and `arm64`.