docs: syncing several computers into one device; changelog for 3.5.0

Add B248 to B252 to the bug ledger, the shared-device feature and schema 44
to the 3.5.0 changelog, and describe the join flow, per-machine profiles,
conflict copy names and the mirror's multi-writer rules in the user guide,
recovery guide, CLI and configuration references and the architecture notes.

docs/CONFIGURATION.md

@@ -14,6 +14,7 @@ and where the client keeps its data.
 | Path | Contents |
 |---|---|
 | `~/.config/proton-drive-linux/config.json` | Settings (this page) |
+| `~/.config/proton-drive-linux/machine-id` | A random id for this machine. It names this machine's profile in a computer's backup that several computers may share |
 | `~/.local/state/proton-drive-linux/cache.db` | Metadata database: names, folder tree, sync state, upload queue, photo index |
 | `~/.local/state/proton-drive-linux/sdk_cache.db` | Encrypted cache of Proton Drive metadata. Safe to delete while the service is stopped |
 | `~/.local/state/proton-drive-linux/control.sock` | Socket the app, tray, prompt and CLI use to reach the service |
@@ -78,7 +79,7 @@ fix it.
 | `upload_limit` | bytes/s | none | General → Network | Shared cap for all uploads. `0` or `null` means none |
 | `download_limit` | bytes/s | none | General → Network | Shared cap for all downloads, reads from the mount included |
 | `ignore_patterns` | list | see below | — | Global ignore rules for synced folders |
-| `device_uid` | string | none | Computers → Continue This Backup Here… | The computer this machine continues. Unset means "match by hostname" |
+| `device_uid` | string | none | Computers → Continue This Backup Here… or Sync With This Computer… | The computer this machine syncs into. Unset means "match by hostname" |
 | `conflict_sweep` | `off` / `report` / `enforce` | `report` | — | See [`conflict_sweep`](#conflict_sweep) |
 | `local_first` | bool | `true` | — | See [`local_first`](#local_first) |
 | `language` | gettext code | system | General → Appearance | Language of the app, tray and prompt, such as `de` or `pt_BR` |