docs: fix/improve developer docs

Author: xmatthias

docs/developer.md

@@ -413,7 +413,11 @@ jupyter nbconvert --ClearOutputPreprocessor.enabled=True --to markdown freqtrade
 To generate backtest outputs, please use the following commands:
 
 ``` bash
+# Assume a dedicated user directory for this output
 freqtrade create-userdir --userdir user_data_bttest/
+# set can_short = True
+sed -i "s/can_short: bool = False/can_short: bool = True/" user_data_bttest/strategies/sample_strategy.py
+
 freqtrade download-data --timerange 20250625-20250801 --config tests/testdata/config.tests.usdt.json --userdir user_data_bttest/ -t 5m
 
 freqtrade backtesting --config tests/testdata/config.tests.usdt.json -s SampleStrategy --userdir user_data_bttest/ --cache none --timerange 20250701-20250801