update documentation for --export-filename

docs/backtesting.md

@@ -62,6 +62,12 @@ Where `-s TestStrategy` refers to the class name within the strategy file `test_
 python3 ./freqtrade/main.py backtesting --export trades
 ```
 
+**Exporting trades to file specifying a custom filename**
+```bash
+python3 ./freqtrade/main.py backtesting --export trades --export-filename=backtest_teststrategy.json
+```
+
+
 **Running backtest with smaller testset**  
 Use the `--timerange` argument to change how much of the testset
 you want to use. The last N ticks/timeframes will be used.

docs/bot-usage.md

@@ -120,6 +120,8 @@ Backtesting also uses the config specified via `-c/--config`.
 ```
 usage: main.py backtesting [-h] [-i TICKER_INTERVAL] [--realistic-simulation]
                            [--timerange TIMERANGE] [-l] [-r] [--export EXPORT]
+                           [--export-filename EXPORTFILENAME]
+
 
 optional arguments:
   -h, --help            show this help message and exit
@@ -137,6 +139,11 @@ optional arguments:
                         run your backtesting with up-to-date data.
   --export EXPORT       export backtest results, argument are: trades Example
                         --export=trades
+  --export-filename EXPORTFILENAME
+                        Save backtest results to this filename requires
+                        --export to be set as well Example --export-
+                        filename=backtest_today.json (default: backtest-
+                        result.json
 ```
 
 ### How to use --refresh-pairs-cached parameter?