docs: add FAQ about resetting a bot's database

Author: xmatthias

docs/faq.md

@@ -133,6 +133,10 @@ This message is a warning that the candles had a price jump of > 30%.
 This might be a sign that the pair stopped trading, and some token exchange took place (e.g. COCOS in 2021 - where price jumped from 0.0000154 to 0.01621).
 This message is often accompanied by ["Missing data fillup"](#im-getting-missing-data-fillup-messages-in-the-log) - as trading on such pairs is often stopped for some time.
 
+### I want to reset the bot's database
+
+To reset the bot's database, you can either delete the database (by default `tradesv3.sqlite` or `tradesv3.dryrun.sqlite`), or use a different database url via `--db-url` (e.g. `sqlite:///mynewdatabase.sqlite`).
+
 ### I'm getting "Outdated history for pair xxx" in the log
 
 The bot is trying to tell you that it got an outdated last candle (not the last complete candle).