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/CONFIGURATION.md

@@ -38,7 +38,7 @@ Inside `content/`:
 > not reached Proton Drive and exists nowhere else.
 
 Everything in these directories except the keyring is **decrypted**. See
-[ARCHITECTURE.md §8](ARCHITECTURE.md#8-threat-model-what-this-client-writes-to-disk-in-plaintext).
+[ARCHITECTURE.md §10](ARCHITECTURE.md#10-threat-model-what-this-client-writes-to-disk-in-plaintext).
 
 ## `config.json`
 
@@ -190,13 +190,16 @@ Command-line flags win over the file: `--gtk`, `--dmenu`, `--fzf` and `--menu`.
 | `PDFS_LOCALEDIR` | app, tray, prompt | Load translations from another directory (development) |
 | `PDFS_STAND_IN_WIDGETS` | app | `1` uses the libadwaita 1.5 fallback widgets on newer systems (development) |
 
-The acceptance suite has its own `PDFS_ACCEPTANCE_*` variables, listed in [TESTING.md](TESTING.md).
+The tests have their own variables (`PDFS_ACCEPTANCE_*`, `PDFS_SIM_*`, `PDFS_MIGRATE_DB`),
+listed in [TESTING.md](TESTING.md).
 
 ## The systemd service
 
-`proton-drive.service` is a user unit. It restarts the service whenever it exits, restarts it when
-it stops answering for two minutes (`WatchdogSec=120`), and caps its memory at 6 GiB. To change
-the unit without editing the packaged file, use a drop-in:
+`proton-drive.service` is a user unit started with your graphical session
+(`WantedBy=graphical-session.target`). It restarts the service whenever it exits, restarts it
+when it stops answering for two minutes (`WatchdogSec=120`), throttles it above 2 GiB of memory
+(`MemoryHigh`) and stops it at 6 GiB (`MemoryMax`). To change the unit without editing the
+packaged file, use a drop-in:
 
 ```bash
 systemctl --user edit proton-drive.service