Reset text/font properties more thoroughly for tooltips+popovers

[cvrebert]

Fixes #15925.
Note: This changes the Tooltip line-height from 1.4 to @line-height-base (1.428571429)
Also enhances the explanatory comments a bit.

[mdo]

This seems like overkill honestly. In Primer, we've gotten away with around half this reset: https://github.com/primer/primer/blob/master/scss/_tooltips.scss. It doesn't cover the same amount of things as Bootstrap might encounter, but it's worked out well so far.

[cvrebert]

I grant you that font-kerning, text-indent, font-variant, and text-emphasis are more on the paranoid side. Would removing those alleviate your concerns?

Other than those, all this PR adds compared to Primer is:

line-break: auto;
word-break: normal;
word-spacing: normal;

which are more vanilla/common properties; I am thinking that hypothetical bugs caused by the lack of these 3 resets would end up being deemed legit bugs? So might as well just fix them now.

[mdo]

Sure, let's roll with those.

[cvrebert]

@mdo Okay, revised. LGTY?

[mdo]

Yup, .