Fix three review findings in help text, example config and terms

The snapshot restore help splits its 40-word permissions sentence in
two. The snapshot-ID case leaves the parenthesis, and the temporary
space requirement gets its own sentence.

configs/config.example.yml gains a commented-out env entry, after
hostname, so its first line, which says the file shows every option,
is true again.

docs/REPOSTRUCTURE.md defines remote key in Terms, before the
marker-file section uses it.

Model: fable-5-1

docs/REPOSTRUCTURE.md

@@ -12,6 +12,8 @@ is a storage service set up in rclone. The layout is the same on all three.
 - **Snapshot**: one backup of the paths listed under one name in the config.
 - **Snapshot ID**: the name of a snapshot, in the form
   `<hostname>_<snapshot-name>_<timestamp>`.
+- **Remote key**: a hash of the snapshot ID. It names the snapshot's directory
+  under `metadata/`. See [Remote keys](#remote-keys).
 - **Chunk**: a piece of a file's content, named by the SHA-256 hash of that
   content.
 - **Blob**: one object under `blobs/`. It holds many chunks.