document grid: string etc. (#1408)

* document grid: string etc.

note: there are tests in aapl-close.ts.

closes #1372

* Update README.md

---------

Co-authored-by: Mike Bostock <[email protected]>

Author: Fil

README.md

@@ -296,7 +296,7 @@ Plot automatically generates [axis](#axis) and optionally [grid](#grid) marks fo
 * *scale*.**tickPadding** - the separation between the tick and its label (in pixels; default 3)
 * *scale*.**tickFormat** - either a function or specifier string to format tick values; see [Formats](#formats)
 * *scale*.**tickRotate** - whether to rotate tick labels (an angle in degrees clockwise; default 0)
-* *scale*.**grid** - if true, draw grid lines across the plot for each tick
+* *scale*.**grid** - whether to draw grid lines across the plot for each tick
 * *scale*.**line** - if true, draw the axis line (only for *x* and *y*)
 * *scale*.**label** - a string to label the axis
 * *scale*.**labelAnchor** - the label anchor: *top*, *right*, *bottom*, *left*, or *center*
@@ -305,7 +305,7 @@ Plot automatically generates [axis](#axis) and optionally [grid](#grid) marks fo
 * *scale*.**ariaLabel** - a short label representing the axis in the accessibility tree
 * *scale*.**ariaDescription** - a textual description for the axis
 
-Top-level options are also supported as shorthand: **grid** (for *x* and *y* only; see [facet.grid](#facet-options)), **label**, **axis**, **inset**, **round**, **align**, and **padding**.
+Top-level options are also supported as shorthand: **grid** (for *x* and *y* only; see [facet.grid](#facet-options)), **label**, **axis**, **inset**, **round**, **align**, and **padding**. If the **grid** option is true, show a grid with the currentColor stroke; if specified as a string, show a grid with the specified stroke color; if an approximate number of ticks, an interval, or an array of tick values, show corresponding grid lines.
 
 ### Projection options