message: aead.h: Clarify the thread safety of the Aead interface
  To make it clear that callers that might need concurrent access to the encryption
  primitive do not need to provide their own locking.
diff: @@ -29,6 +29,8 @@ namespace tink {
  // chosen ciphertext attacks. Encryption with associated data ensures
  // authenticity and integrity of that data, but not its secrecy.
  // (see RFC 5116, https://tools.ietf.org/html/rfc5116)
  +//
  +// Implementations are expected to be thread safe.
  class Aead {
  public:
  // Encrypts 'plaintext' with 'associated_data' as associated data,
  