docs: describe local-first changes and sync issues for users

The user guide said only writes were queued. Since 3.0.0 creating,
renaming and deleting through the mount return at once too, and changes
Drive refuses become sync issues. Add a section on changes waiting for
Proton Drive: retry, export, discard, and the two cases that land under
another name or place instead of failing.

Troubleshooting covers changes that have not reached other devices yet,
`pdfs sync issues` and the local_first fallback, and names the real
error for symlinks and hard links (EPERM, not EOPNOTSUPP). CONFIGURATION
lists MemoryHigh and the test variables; CLI notes that `sync export`
takes a folder.

docs/CLI.md

@@ -83,7 +83,7 @@ commands are for scripts and for cases where the mount is not available.
 | `pdfs sync limit [--up RATE] [--down RATE]` | Cap bandwidth, for example `500K` or `2M` per second. `0` removes a cap. Without options, prints the current caps. |
 | `pdfs sync queue` | List changes that have not reached Proton Drive yet. |
 | `pdfs sync issues` | List queued changes Proton Drive refused or that keep failing, and what to do. |
-| `pdfs sync export ID DEST` | Save a copy of a queued upload's content. The upload stays queued. |
+| `pdfs sync export ID DEST` | Save a copy of a queued upload's content. `DEST` may be a folder; an existing file is never overwritten. The upload stays queued. |
 | `pdfs sync discard ID [--yes]` | Drop a queued change and undo it here: a new file or folder goes, anything else goes back to what Proton Drive has. Content not exported first is lost. |
 | `pdfs sync retry [ID]` | Retry one queued operation, or every failed one. |
 | `pdfs sync restore [--device UID] [--yes]` | Re-attach this computer's backed-up folders to local folders and download them. `--device` restores another computer's folders. See [RECOVERY.md](RECOVERY.md). |