Fix clippy

Author: viirya

arrow-buffer/src/buffer/immutable.rs

@@ -74,6 +74,12 @@ impl Buffer {
     /// Returns the offset, in bytes, of `Self::ptr` to `Self::data`
     ///
     /// self.ptr and self.data can be different after slicing or advancing the buffer.
+    ///
+    /// # Safety
+    ///
+    /// This function is unsafe as it uses unsafe function `offset_from`. Under certain
+    /// conditions, this function can cause undefined behavior. See the documentation of
+    /// `offset_from` for more information.
     pub unsafe fn ptr_offset(&self) -> usize {
         self.ptr.offset_from(self.data.ptr().as_ptr()) as usize
     }