Document BG Update Delay setting

[bjorkert]

Expands the Advanced settings section to explain the BG Update Delay setting, which was previously only listed as a bare bullet.

Adds:

• What it does — a small timing buffer (in seconds) that LoopFollow waits after a new reading is expected before fetching it from the data source, giving the value time to arrive at Nightscout / Dexcom Share before polling.
• The default (10 seconds) and range (1–30).
• Why lowering it doesn't reliably make updates faster, and that it mostly adds extra polls (battery/network).
• That the delay isn't used when Nightscout WebSocket is active in the foreground (values are pushed), and only affects background polling — linked to the existing WebSocket section.