message: fix fastfield doc
diff: //!
  //!
  //! Fields have to be declared as `FAST` in the schema.
  -//! Currently supported fields are: u64, i64 and f64.
  -//! u8 are supported for advanced usage.
  +//! Currently supported fields are: u64, i64, f64 and bytes.
  //!
  -//! They are stored in a bit-packed fashion so that their
  -//! memory usage is directly linear with the amplitude of the
  +//! u64, i64 and f64 fields are stored in a bit-packed fashion so that
  +//! their memory usage is directly linear with the amplitude of the
  //! values stored.
  //!
  //! Read access performance is comparable to that of an array lookup.
  