Clarify documentation for "overlaying"
Clarify documentation for "overlaying"

related: #3074
etpinard/plotly.js,asolagmbh/plotly.js,aburato/plotly.js,plotly/plotly.js,etpinard/plotly.js,etpinard/plotly.js,aburato/plotly.js,plotly/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,asolagmbh/plotly.js