message: Add documentation for `len` field on `Storage`
  Because the purpose of the `len` field wasn't obvious and collided with
  other uses (like Vec::len()), some additional documentation has added to
  make things a little easier to understand.
diff: @@ -20,6 +20,13 @@ pub struct Storage<T> {
  inner: Vec<T>,
  zero: usize,
  visible_lines: Line,
  +
  + /// Total number of lines currently active in the terminal (scrollback + visible)
  + ///
  + /// Shrinking this length allows reducing the number of lines in the scrollback buffer without
  + /// having to truncate the raw `inner` buffer.
  + /// As long as `len` is bigger than `inner`, it is also possible to grow the scrollback buffer
  + /// without any additional insertions.
  #[serde(skip)]
  len: usize,
  }
  