[Legend] Instructions descriptions in tooltip

[gvnmagni]

In order to guide users into the available interactions on top of legends we are going to use the browser tooltip to describe all the possibilities.

This following image shows the expected list of description for each case.

Additional note: hovering a hidden series would display only one line saying "Click to show" since the "⌘ + click" would perform the same action, we will make sure though that even this redundant information will be read by screenreaders.

@dej611 tagging you only for visibility so you don't have to look for this issue.

[gvnmagni]

Related to this meta issue: elastic/kibana#176583