Docs: fix mutableArrayData comments (#10326)

# Which issue does this PR close?

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- Closes #9398.

# Rationale for this change
the comments on
[arrow-rs/arrow-data/src/transform/mod.rs](https://github.com/apache/arrow-rs/blob/d6168e526aae79d6fbafe8c11062b5f834021052/arrow-data/src/transform/mod.rs#L389-L401)
were misleading/incorrect
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# What changes are included in this PR?

- updated DocTest to be accurate
- updated `MutableArrayData::new()` to correctly use slots instead of
bytes for allocation
- see comment on issue ->
#9398 (comment)
 
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# Are these changes tested?
yes
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# Are there any user-facing changes?
yes, users will have better doc comments.
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---------

Co-authored-by: Richard Baah <richardbaah@MacBook-Air-de-Richard.local>

Author: Rich-T-kid

arrow-data/src/transform/mod.rs

@@ -118,7 +118,7 @@ fn build_extend_null_bits(array: &ArrayData, use_nulls: bool) -> ExtendNullBits<
 /// use arrow_data::transform::MutableArrayData;
 /// use arrow_schema::DataType;
 /// fn i32_array(values: &[i32]) -> ArrayData {
-///   ArrayData::try_new(DataType::Int32, 5, None, 0, vec![Buffer::from_slice_ref(values)], vec![]).unwrap()
+///   ArrayData::try_new(DataType::Int32, values.len(), None, 0, vec![Buffer::from_slice_ref(values)], vec![]).unwrap()
 /// }
 /// let arr1  = i32_array(&[1, 2, 3, 4, 5]);
 /// let arr2  = i32_array(&[6, 7, 8, 9, 10]);
@@ -128,7 +128,7 @@ fn build_extend_null_bits(array: &ArrayData, use_nulls: bool) -> ExtendNullBits<
 /// // Copy the first 3 elements from arr1
 /// mutable.extend(0, 0, 3);
 /// // Copy the last 3 elements from arr2
-/// mutable.extend(1, 2, 4);
+/// mutable.extend(1, 2, 5);
 /// // Complete the MutableArrayData into a new ArrayData
 /// let frozen = mutable.freeze();
 /// assert_eq!(frozen, i32_array(&[1, 2, 3, 8, 9, 10]));
@@ -397,13 +397,12 @@ impl<'a> MutableArrayData<'a> {
     ///
     /// # Arguments
     /// * `arrays` - the source arrays to copy from
-    /// * `use_nulls` - a flag used to optimize insertions
-    ///   - `false` if the only source of nulls are the arrays themselves
-    ///   - `true` if the user plans to call [MutableArrayData::extend_nulls].
-    /// * capacity - the preallocated capacity of the output array, in bytes
+    /// * `use_nulls` - a flag indicating whether the caller intends to call `extend_nulls`.
+    ///   Note: null-handling is enabled automatically if any source array contains nulls.
+    /// * `capacity` - the preallocated capacity of the output array, in slots (number of elements)
     ///
-    /// Thus, if `use_nulls` is `false`, calling
-    /// [MutableArrayData::extend_nulls] should not be used.
+    /// if `use_nulls` is `false` and no source arrays contains nulls, calling
+    /// [MutableArrayData::extend_nulls] or [MutableArrayData::try_extend_nulls] will panic.
     pub fn new(arrays: Vec<&'a ArrayData>, use_nulls: bool, capacity: usize) -> Self {
         Self::with_capacities(arrays, use_nulls, Capacities::Array(capacity))
     }