[protocol spec] Rename §4.19.3 "Decryption using a Full Viewing Key (Sapling and Orchard)"

[upbqdn]

Motivation

When reading §4.19.3 "Decryption using a Full Viewing Key (Sapling and Orchard)" in the protocol specification, I didn't notice any use of the full viewing key (specifically the ak and nk parts) in that section. The algorithm described there uses only the outgoing viewing key.

Suggested Solution

Mention outgoing viewing keys in the name of the section instead of full viewing keys.

[daira]

This did not make it into 2023.4.0.