docs(reference): add auto_update, drop drifted line numbers, fix UI and service details

- CONFIG_REFERENCE: document the top-level auto_update.enabled key (read
  by web_interface/auto_update.py and src/auto_update_setup.py); replace
  drifted file:line references with function names; the template's
  dim_schedule mode is "global".
- ADVANCED_FEATURES: core does not read a per-plugin background_service
  block (the sports plugins read their own), and priority is "higher
  number = higher priority" on FetchRequest but not used for ordering.
- WEB_INTERFACE_GUIDE: the General tab toggle is "Web Display Autostart"
  (web interface service), brightness is 1-100, and config paths are
  relative to the LEDMatrix folder, not /config.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/WEB_INTERFACE_GUIDE.md

@@ -95,7 +95,8 @@ Configure basic system settings:
   plugins
 - **Plugin System Settings** — including the `plugins_directory` (default
   `plugin-repos/`) used by the plugin loader
-- **Autostart** options for the display service
+- **Web Display Autostart** — whether the web interface service starts
+  with the system (`web_display_autostart`)
 - **Automatic updates** — once a week, update LEDMatrix and every installed
   plugin with a newer version. Off by default. Runs 2–5 AM local time when
   possible, otherwise within a day of being due. The last result and next
@@ -246,7 +247,7 @@ View real-time system logs:
 ### Changing Display Brightness
 
 1. Open the **Display** tab
-2. Adjust the **Brightness** slider (0–100)
+2. Adjust the **Brightness** slider (1–100)
 3. Click **Save**
 4. Click **Restart Display Service** on the **Overview** tab
 
@@ -428,10 +429,10 @@ The web interface uses modern web technologies:
 
 ### File Locations
 
-**Configuration:**
-- Main config: `/config/config.json`
-- Secrets: `/config/config_secrets.json`
-- WiFi config: `/config/wifi_config.json`
+**Configuration** (relative to the LEDMatrix folder, e.g. `~/LEDMatrix`):
+- Main config: `config/config.json`
+- Secrets: `config/config_secrets.json`
+- WiFi config: `config/wifi_config.json`
 
 **Logs:**
 - Display service: `sudo journalctl -u ledmatrix -f`
@@ -444,7 +445,7 @@ The web interface uses modern web technologies:
   the Plugin Store install flow and the schema loader additionally
   probe `plugins/` so dev symlinks created by
   `scripts/dev/dev_plugin_setup.sh` keep working.
-- Plugin config: `/config/config.json` (per-plugin sections)
+- Plugin config: `config/config.json` (per-plugin sections)
 
 ---