Add documentation to libsodium bindings API

[ionspin]

Describe the bug Documentation of the Libsodium bindings API is either completely missing, or a direct copy from libsodium website

To Reproduce Look at the API documentation

Expected behavior A documentation explaining what a method/function does precisely

Platform

• All

Tasks: Add .md description for each functionality group and add documentation for each function in the group

• [x] aead
• [x] auth
• [x] box
• [x] generichash
• [ ] hash
• [ ] kdf
• [ ] keyexchange
• [ ] pwhash
• [ ] scalarmult
• [ ] secretbox
• [ ] secretstream
• [ ] shortinputhash
• [ ] signature
• [ ] stream
• [ ] util