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

从 examples/styles.ipynb 开始，检查 charles、yahoo 和 starsandstripes 样式示例。确认成交量颜色与蜡烛颜色之间的关系，并记录 vcdopcod 功能和观察到的行为，以便清楚地说明样式之间的差异。