Update doc on label types for TooltipItem

[nagix]

label and value in TooltipItem are the return values from scale.getLabelForIndex which can be number or string.

Fixes #5830

[simonbrunel]

@nagix @kurkle shouldn't we instead force label and value to be strings?

At some point we will introduce a formatter to be able to suffix the value (e.g. 42 -> '42%') in the tooltip, so to me it would make sense that these tooltip item label and value are always strings.

[benmccann]

I was debating that as well. That approach would make sense to me

[kurkle]

I agree they should be strings. That is not the case (yet) though, and these changes reflect current situation.
When/if I get proposal for formatter option done, these can be re-adjusted in that PR (or whatever PR that provides the conversion to strings).

[simonbrunel]

If users expect label and value to be numbers (thus to be the original data), then it may be a breaking change to automatically convert it to string later. Since it's 2 new properties, it may be better to force a string before releasing 2.8?

[kurkle]

For these new ones, yes I think they should be formatted and/or forced to string before 2.8 release.
Those old ones will most likely have to stay as they are.
.. so maybe forcing these new ones to string right now would not be a bad start.

[nagix]

Now createTooltipItem sets label and value to string values. I have also added a few tests.

[kurkle]

Kudos for adding tests!