Document the SDF spread further.
diff --git a/include/freetype/ftdriver.h b/include/freetype/ftdriver.h
index 4c83f73..dd31626 100644
--- a/include/freetype/ftdriver.h
+++ b/include/freetype/ftdriver.h
@@ -830,7 +830,8 @@
* the result clamped to the 8-bit range [0..255]. Therefore, 'spread'
* is also the maximum euclidean distance from the edge after which the
* values are clamped. The spread is specified in pixels with the
- * default value of 8.
+ * default value of 8. For accurate SDF texture mapping (interpolation),
+ * the spread should be large enough to accommodate the target grid unit.
*
* @example:
* The following example code demonstrates how to set the SDF spread
