Add .h1 – .h6 example to docs

[cvrebert]

Refs #18491, http://v4-alpha.getbootstrap.com/content/typography/#headings

[patrickhlauke]

related to this advice

for when you want to match the font styling of a heading but still want your text to be displayed inline

though, I'm not particularly thrilled about the supposed rationale. If something's semantically a heading, it should always use <h1> through <h6>. If it's only about having it "displayed inline" then it should be a case of still using the correct heading element and adding a display:inline style, and not to just use an arbitrary element and slapping a class="h1" or similar on it.

maybe the wording just needs to be changed to something more like

for when you want an arbitrary piece of text to match the font styling of a heading

[cvrebert]

Yeah, we could probably stand to tweak that.
The <h3 class="h4"> use case (i.e. this is logically a level 3 heading in the outline tree, but the normal <h3> styling would be insufficiently/excessively prominent) isn't covered by the current rationale either.

and adding a display:inline style

Perhaps that deserves a utility class?

[patrickhlauke]

i think (though I'm just going by my own practice, so take with a pinch of salt) that the "i want a heading, but have it shown inline" use case it pretty marginal/edge. also, personally not a fan of classes that only really set a single property, but that's just me.

[mdo]

Fixed in b479efb.

Dropped that whole inline business—no idea what that was about. This is just a set of utilities to apply an HTML element's default styles to any other element.