improve notched attr description

Author: etpinard

src/traces/box/attributes.js

@@ -100,7 +100,12 @@ module.exports = {
         role: 'style',
         editType: 'calc',
         description: [
-            'Determines whether or not notches should be drawn.'
+            'Determines whether or not notches are drawn.',
+            'Notches displays a confidence interval around the median.',
+            'We compute the confidence interval as median +/- 1.57 / IQR * sqrt(N),',
+            'where IQR is the interquartile range and N is the sample size.',
+            'If two boxes\' notches do not overlap there is 95% confidence their medians differ.',
+            'See https://sites.google.com/site/davidsstatistics/home/notched-box-plots for more info.'
         ].join(' ')
     },
     notchwidth: {