Clarify absence of offline functionality in XRPL documentation

[xVet]

Because of confusion between XRP Ledger and Mandla App capabilities, adjusted and refined the wording to make it explicit which feature belongs to which system. Else it appeared the XRP Ledger has offline payment capabilities which is obviously not true.

[hazardcookie]

Thanks for doing this!

[amarantha-k]

Thanks for proposing these updates, @xVet! Looks good to merge once the suggestions from @intelliot are committed.