len is not a parameter of MutableArrayData::extend

[HaoYang670]

Describe the bug
Noticed while working on #1230 .
The document of MutableArrayData::extend contains a weird value len which is not a parameter of this function.

To Reproduce
Steps to reproduce the behavior:

Expected behavior
Replace len by end - start in the document, or change the function's API

Additional context

    /// Extends this [MutableArrayData] with elements from the bounded [ArrayData] at `start`
    /// and for a size of `len`.
    /// # Panic
    /// This function panics if the range is out of bounds, i.e. if `start + len >= array.len()`.
    pub fn extend(&mut self, index: usize, start: usize, end: usize) {
        let len = end - start;
        (self.extend_null_bits[index])(&mut self.data, start, len);
        (self.extend_values[index])(&mut self.data, index, start, len);
        self.data.len += len;
    }

[alamb]

I think fixing the doc comment would be the best approach here