Labels should not include the element's role

[daniel-montalvo]

From @sloandr in #452

Labels like “image of xyz” “link to zyx” “xyz button” are redundant. ATs announce the role’s name (image, link, button, etc.) when using the correct role.

We currently only mention this in Developer Modules, Images, Text Alternatives, teaching ideas, first bullet:

[...] State why it is not necessary to include the word “image” in the text alternative because the HTML element img is already announced by assistive technologies. [...]

What do we need to have in designers and authors? Is this just restricted to images or we need to include this in other places where we cover labels?