Document changes to 'Encrypted Streams' API made in #39

[andreiled]

Document changes to 'Encrypted Streams' API made in #39

1. Update crypto_secretstream_xchacha20poly1305_init_* methods documentation to indicate that they both asynchronously return a Stream Encryptor/Decryptor object completely encapsulating the backend-specific state
2. Document properties and methods available in the Stream Encryptor/Decryptor objects
3. Remove documentation for private methods that can no longer be used directly as they require access to the underlying backend-specific state encapsulated by the Stream Encryptor/Decryptor objects:
   • crypto_secretstream_xchacha20poly1305_push
   • crypto_secretstream_xchacha20poly1305_pull
   • crypto_secretstream_xchacha20poly1305_rekey
4. Update sample for stream encryption/decryption

[andreiled]

• Option A: https://github.com/andreiled/sodium-plus/blob/3ca550ab6f8160f676b05e59b5f21305f59507f9/docs/SodiumPlus/encrypted-streams.md
• Option B: https://github.com/andreiled/sodium-plus/blob/a245226104ca5c3b71b708c495f1d564802bc65b/docs/SodiumPlus/encrypted-streams.md

[andreiled]

@paragonie-scott, could you take a look at this documentation follow-up for the previously merged #39?

[stevenroussey-privicy]

The typescript return types need some updating as well.