Layout attribute docs: expand description for minexponent

Author: ignamv

src/plots/cartesian/layout_attributes.js

@@ -687,7 +687,8 @@ module.exports = {
         role: 'style',
         editType: 'ticks',
         description: [
-            'Hide SI prefix for 10^n if |n| is below this number'
+            'Hide SI prefix for 10^n if |n| is below this number.'
+            'This only has an effect when `tickformat` is *SI* or *B*.'
         ].join(' ')
     },
     separatethousands: {