docs: data messages documentation

proto/src/message/primitive/pgstring.rs

@@ -4,6 +4,7 @@ use bincode::enc::Encoder;
 use bincode::error::{DecodeError, EncodeError};
 use bincode::{BorrowDecode, Decode, Encode};
 
+/// PostgreSQL format of string encoded as a null-terminated string.
 #[derive(Debug, Clone, BorrowDecode)]
 pub struct PgString(String);