Document **vcdopcod** (Volume Color Depends on Price Change on Day) feature of mplfinance styles.

Describe the bug

Using style as charles or yahoo with candle and volume chart, some of their color don't match.
This is not happening to starsandstripes styled chart.

To Reproduce

Please take a look at charles or yahoo styled chart in the example page.
https://github.com/matplotlib/mplfinance/blob/master/examples/styles.ipynb

The second candle from the left is red in the chart but its volume bar is green.
The fifth candle from the left is red but its volume bar is green.

This is happening to my project as well.

Cheers

examples/styles.ipynb から始めて、charles、yahoo、starsandstripes のスタイル例を調べてください。出来高の色がローソク足の色とどのように関係するかを確認し、vcdopcod 機能と確認された動作を文書化して、スタイルの違いが明確になるようにしてください。