Minor: more comments for RecordBatch.get_array_memory_size() (#6607)

* doc RecordBatch size

* Update arrow-array/src/record_batch.rs

Co-authored-by: Raphael Taylor-Davies <1781103+tustvold@users.noreply.github.com>

---------

Co-authored-by: Raphael Taylor-Davies <1781103+tustvold@users.noreply.github.com>

arrow-array/src/record_batch.rs

@@ -469,6 +469,11 @@ impl RecordBatch {
     }
 
     /// Returns the total number of bytes of memory occupied physically by this batch.
+    ///
+    /// Note that this does not always correspond to the exact memory usage of a
+    /// `RecordBatch` (might overestimate), since multiple columns can share the same
+    /// buffers or slices thereof, the memory used by the shared buffers might be
+    /// counted multiple times.
     pub fn get_array_memory_size(&self) -> usize {
         self.columns()
             .iter()