document x-axis customization

### Description
The x-axis has data labels on both the top and the bottom which looks a bit cluttered.

### Expected behavior
It should be easy to customize the x-axis, please create documentation for.

1. removing/enabling the top label [DONE] `coarse.time=TRUE||FALSE`
2. removing/enabling the bottom label without extra whitespace remaining [WORKAROUND] `format.labels <- ' '`
3. removing/enabling tick marks for top x-axis
4. removing/enabling tick marks for bottom x-axis

### Minimal, reproducible example

```r
### basic
getSymbols("GM")
chart_Series(GM, subset='2012::2019')
```

```r
### blanking bottom label, though the whitespace still persists
getSymbols("GM")
ct <- chart_theme()
ct$format.labels <- ' '
chart_Series(GM, subset='2012::2019', theme=ct)
```

```r
getSymbols("GM")
ct <- chart_theme()
ct$coarse.time <- FALSE ### removes the label above the x-axis
chart_Series(GM, subset='2012::2019', theme=ct)
```

Start with the chart_Series() and chart_theme() examples in the issue, using coarse.time and format.labels as the documented settings. Check how top and bottom labels and tick marks are currently controlled, then document each option, including removal without leftover whitespace. Done means the four requested customizations have clear, working examples.

Written by the indexing model from the issue text.