Update custom-buttons.md

Bolded phrases and additional explanation in the "methods" section.

Author: SylwiaOliwia2

doc/python/custom-buttons.md

@@ -35,9 +35,9 @@ jupyter:
 
 #### Methods
 The [updatemenu method](https://plot.ly/python/reference/#layout-updatemenus-items-updatemenu-buttons-items-button-method) determines which [plotly.js function](https://plot.ly/javascript/plotlyjs-function-reference/) will be used to modify the chart. There are 4 possible methods:
-- `"restyle"`: modify data or data attributes
-- `"relayout"`: modify layout attributes
-- `"update"`: modify data **and** layout attributes
+- `"restyle"`: modify **data** or data attributes
+- `"relayout"`: modify **layout** attributes
+- `"update"`: modify **data and layout** attributes; combination of `"restyle"` and `"relayout"`
 - `"animate"`: start or pause an [animation](https://plot.ly/python/#animations))