message: Add beginner-friendly explanation of AEADs to PRIMITIVES docs.
diff: @@ -91,12 +91,17 @@ Hybrid Encryption | ECIES with AEAD and HKDF
  AEAD primitive (Authenticated Encryption with Associated Data) provides
  functionality of symmetric authenticated encryption. Implementations of this
  -primitive are secure against adaptive chosen ciphertext attacks. When
  -encrypting a plaintext one can optionally provide _associated data_ that should
  -be authenticated but not encrypted. That is, the encryption with associated data
  -ensures authenticity (ie. who the sender is) and integrity (ie. data has not
  -been tampered with) of that data, but not its secrecy
  -(see [RFC 5116](https://tools.ietf.org/html/rfc5116)).
  +primitive are secure against adaptive chosen ciphertext attacks.
  +
  +When encrypting a plaintext one can optionally provide _associated data_ that
  +should be authenticated but not encrypted. That is, the encryption with
  +associated data ensures authenticity (ie. who the sender is) and integrity (ie.
  +data has not been tampered with) of that data, but not its secrecy (see
  +[RFC 5116](https://tools.ietf.org/html/rfc5116)). This is often used for binding
  +encryptions to a context. For example, in a banking database the contents of a
  +row (e.g. bank account balance) can be encrypted using the customer's id as
  +_associated data_. This would prevent swapping encrypted data between customers'
  +records.
  Minimal properties:
  