nextcloud: document how to get debugging logs

docs/nextcloud.md

@@ -0,0 +1,19 @@
+# Nextcloud debugging
+
+Set loglevel to `0` for debug logs:
+
+```nix
+services.nextcloud.settings.loglevel = 0;
+```
+
+Then, logs appear in the `phpfpm-nextcloud.service` logs:
+
+```bash
+sudo journalctl -fu phpfpm-nextcloud
+```
+
+Make sure to set the loglevel back to the default `2` warning after debugging:
+
+```nix
+services.nextcloud.settings.loglevel = 2;
+```