Deprecated `text` / use `hovertext` for traces that don't support on-graph text

[etpinard]

Attribute hovertext got added (first in #1523) for traces that support on-graph text. There are all the scatter* traces, pie and bar. Traces that support hover but do not support on-graph text currently use text to set the "hover" text. As @nicolaskruchten points out, this is a little confusing.

So, should we deprecate text (while renaming hovetext) in traces that do not support on-graph text?

[nicolaskruchten]

We could start by just adding hovertext in the relevant places... that increases consistency and breaks nothing :)

[alexcjohnson]

Agreed that the end state should be no text attributes that only ever appear on hover. But we should look at the traces that currently have no on-graph text and figure out which ones we can imagine adding it to in the future, before actually removing the text attribute (in a major bump).

But yes, step 1 can be adding hovertext everywhere it's relevant.