Add hover text with explanations to html report

[gvanrossum]

It would be nice if text colored red or yellow in the report showed the reason for this color when you hover the mouse over a highlighted area.

[dosisod]

Is this still an issue? I saw that there is indeed hover text support for HTML reports (at least in the code snippet sections).

[ilinum]

@dosisod you're correct! This was added in #3798!