MPT/token doc follow-ups

[mDuo13]

Follow-up for open questions on #3312:

• what actually is the max number of entries in an owner directory?
• can accepting a token escrow cause your line to go above the limit, similar to cashing a check? (related: Fix documentation of TokenEscrow amendment #3260)
• consider rewriting section on stablecoins to clarify fractional reserve

[maria-robobug]

Adding context for the fractional reserve part.

Fractional reserve stablecoins are a real (if somewhat frowned-upon) possibility. For a long time, people have suspected that USDT (Tether) is not fully backed—but as long as they have enough USD to process withdrawals, it remains stable.

We should explain the difference between the more generally understood definition of stablecoin, and fractional stablecoins and the risk associated with it.