document rounding

Author: Fil

docs/interactions/brush.md

@@ -233,7 +233,7 @@ The brush value dispatched on [_input_ events](#input-events). When the brush is
 - **filter** - a function to test whether a point is inside the selection
 - **pending** - `true` during interaction; absent when committed
 
-By convention, *x1* < *x2* and *y1* < *y2*. The brushX value does not include *y1* and *y2*; similarly, the brushY value does not include *x1* and *x2*.
+By convention, *x1* < *x2* and *y1* < *y2*. The brushX value does not include *y1* and *y2*; similarly, the brushY value does not include *x1* and *x2*. Values are automatically rounded to the optimal precision that distinguishes neighboring pixels.
 
 The **pending** property indicates the user is still interacting with the brush. To skip intermediate values and react only to committed selections: