Document --exchange can accept multiple commodities

The --exchange option can accept multiple comma-separated commodities,
but this is only mentioned in one place where --exchange is documented,
and notably not in the Texinfo manual.

Examples made with the following ledger file:

```
P 2022-06-28 USD 54.81 PHP
P 2022-06-28 EUR 58.01 PHP

2022-06-28 * Opening Balances
    Equity:Opening Balances
    Assets:USD Bank                          100 USD
    Assets:EUR Bank                          100 EUR
    Assets:PHP Bank                          100 PHP
```

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