Clarify histnorm description

This is consistent with https://plot.ly/python-api-reference/generated/plotly.express.histogram.html#plotly.express.histogram and https://plot.ly/python/reference/#histogram-histnorm.

Author: potpath

doc/python/histograms.md

@@ -88,7 +88,7 @@ fig.show()
 
 #### Type of normalization
 
-The default mode is to represent the count of samples in each bin. With the `histnorm` argument, it is also possible to represent the percentage or fraction of samples in each bin (`histnorm='percent'` or `probability`), or a density histogram (the sum of bars is equal to 100, `density`), or a probability density histogram (sum equal to 1, `probability density`).
+The default mode is to represent the count of samples in each bin. With the `histnorm` argument, it is also possible to represent the percentage or fraction of samples in each bin (`histnorm='percent'` or `probability`), or a density histogram (the sum of all bar areas equals the total number of sample points, `density`), or a probability density histogram (the sum of all bar areas equals 1, `probability density`).
 
 ```python
 import plotly.express as px