Clarify entropy source for snap_getBip44Entropy

[SebastienGllmt]

Right now Snaps offers snap_getBip44Entropy and snap_getBip32Entropy. However, It's not mentioned what the source of this entropy is. The documentation seems to indicate that it's the same root entropy the user used when setting up Metamask itself, but I think the source should be clarified given snap_getentropy explicitly states that at least for itself, it's snap-specific entropy

We also do not explain how BIP32 and BIP44 are related or why you should use one or the other.

We also do not explain the pitfall of using an unknown coinType or how to add a coinType to SLIP44

[Montoya]

snap_getEntropy is specific to the Snap, snap_getBip44Entropy and snap_getBip32Entropy are not.