[Enhancement] Mention `all_elements` feature in the Docs

[wiseaidev]

What is wrong?

This is not a feature request, but rather a suggestion. While using the ErrorBar struct, I didn't know that it requires the all_elements feature to be included. So, I would like to suggest updating the docs to mention this feature. Otherwise, beginners to Plotters will spend a lot of time trying to figure out why ErrorBar cannot be imported.