Add doc(cfg) to read_from_io and write_to_io (#2786)

Added `#[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]` to `FromBytes::read_from_io` and `IntoBytes::write_to_io` in `src/lib.rs` to ensure feature requirements are visible in the documentation.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

src/lib.rs

@@ -4665,6 +4665,7 @@ pub unsafe trait FromBytes: FromZeros {
     /// let header = BitmapFileHeader::read_from_io(&mut file).unwrap();
     /// ```
     #[cfg(feature = "std")]
+    #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
     #[inline(always)]
     fn read_from_io<R>(mut src: R) -> io::Result<Self>
     where
@@ -5401,6 +5402,7 @@ pub unsafe trait IntoBytes {
     /// assert_eq!(dst, [255, 255]);
     /// ```
     #[cfg(feature = "std")]
+    #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
     #[inline(always)]
     fn write_to_io<W>(&self, mut dst: W) -> io::Result<()>
     where