chore: Document encryption context interoperability for invalid UTF-8

Author: Lucas McDonald

include/aws/cryptosdk/materials.h

@@ -136,6 +136,15 @@ struct aws_cryptosdk_enc_request {
      * The encryption context for this message. CMMs are permitted to modify this
      * hash table in order to inject additional keys or otherwise modify the encryption
      * context.
+     *
+     * The values provided to the encryption context SHOULD be UTF-8 encoded bytes.
+     * You may provide invalid UTF-8 characters in the encryption context,
+     * but this is discouraged.
+     * Messages with invalid UTF-8 characters will not be interoperable
+     * with other language implementations of the encryption SDK.
+     * The encryption SDK for C will allow writing and reading messages with invalid UTF-8, but
+     * the encryption SDK in other languages will neither read nor write
+     * messages with invalid UTF-8.
      */
     struct aws_hash_table *enc_ctx;
     /**