More clarification on entropy sources for Trezor/Ledger HSMs

[lukechilds]

The disadvantages section of both Trezor and Ledger say that the entropy source is controlled entirely by ARM:

book/content/docs/security/personal-hsms/ledger.md

Line 20 in 1845c4c

* Entropy source is controlled entirely by ARM

book/content/docs/security/personal-hsms/trezor.md

Line 15 in 1845c4c

* Entropy source is controlled entirely by ARM

I think it would be beneficial to clarify that the user can use their own external entropy as a source by importing a BIP39 mnemonic seed phrase.

If this has been carefully generated via a scheme like diceware then it's literally just using the universe as it's entropy source.

In my opinion this is a huge advantage of these two devices.

[RyanSquared]

@lrvick I know that BIP39 seed phrasing is something you were interested in. Did you know about this?