Update ZIP 32 to document the transparent key hierarchy, including the use of index 2 at the scope/change level

[daira]

ZIP 32 previously described only the shielded key hierarchy, on the basis that the transparent usage was the same as BIP 44. Since we're now diverging from BIP 44 in the use of scope 2 for ephemeral transparent addresses, I think it makes most sense to have ZIP 32 cover transparent, since that is where someone would naturally look.

See also zcash/librustzcash#1401.