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/TROUBLESHOOTING.md

@@ -113,29 +113,41 @@ pdfs refresh Documents
 **"Permission denied" when writing** — the item is in a share where you are a viewer, or under
 `Shared with me/`, which is read-only as a folder. Check your role with `pdfs shared-with-me`.
 
-**A program says "Operation not supported"** — Proton Drive cannot store symbolic links, hard
-links, device files or FIFOs. The error is intentional. Use a synced folder for
-projects that need them, or exclude those paths.
+**`ln` or another program fails with "Operation not permitted"** — Proton Drive cannot store
+symbolic links, hard links, device files or FIFOs, so creating one fails at once. The error is
+intentional. Use a synced folder for projects that need them, or exclude those paths.
 
 ## Uploads and sync
 
-**Something is stuck uploading** — list the queue and retry:
+**A change shows here but not on other devices** — changes are recorded on this computer first
+and sent from a queue. List what is still waiting, and what needs attention:
 
 ```bash
 pdfs sync queue
+pdfs sync issues
+```
+
+The app shows the same on **Sync → Overview**. A change waits while the service is offline or
+sync is paused (`pdfs status`), and backs off after a failure. To retry at once:
+
+```bash
 pdfs sync retry           # every failed operation
 pdfs sync retry 42        # one operation
 ```
 
-The app shows the same list on **Sync → Overview**. An operation that keeps failing shows its
-error there. A change Proton Drive will not take can be dropped instead, which undoes it here;
-save a file's content first if you want to keep it:
+A change Proton Drive refuses (storage full, access taken away, folder gone) is listed by
+`pdfs sync issues` with what to do. It stays queued until you fix the cause or drop it. Dropping
+it undoes it here; save a file's content first if you want to keep it:
 
 ```bash
 pdfs sync export 42 ~/Desktop/
 pdfs sync discard 42
 ```
 
+If you suspect the local-first behaviour of 3.0.0 itself, `"local_first": false` in `config.json`
+makes the Proton Drive folder wait for Proton Drive again while online (see
+[CONFIGURATION.md](CONFIGURATION.md#local_first)), and please report the problem.
+
 **A file in a synced folder does not upload** — check that:
 
 - it is not matched by `.pdfsignore` or `ignore_patterns`;