Closes #16151 by merging a rebased version of it that adds docs and 1 more assertion.

This documents the limitation discussed in https://github.com/twbs/bootstrap/issues/12126:

> When using column reordering and using the affix-plugin on an element positioned with push, the affix plugin doesn't work correctly - the element affected by the affix plugin is positioned to the left even when the element is in a column positioned on the right.

Seems to affect Safari (tested in version 8) only.

That issue has (unfortunately) been closed, so this at least documents the behavior.

As per discussion on https://github.com/w3c/wai-wcag-quickref/issues/2
and #13554

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Before, most callouts hierarchically came under Examples > Four
directions, which is not appropriate. Now they're under Markup

Include callout explicitly mentioning tooltips should only be added to
focusable elements. Closes #16134

Refs #15947

Closes #15915 by merging it.

X-Ref #15742

cc: @mdo

Fixes #15106.
Closes #15910.

Fixes #15882 — Use an ID as the example for Scrollspy so that it is more clear that this is for use with a single nav.

Closes #15725 by merging a tweaked version of it.

[Fixes #15766] Need Clearer Affix plugin documentation

Remove trailing whitespace

Update the documentation

Refs https://github.com/twbs/bootstrap-blog/commit/1c94b7791f8a24a18f08aba40c38d519b873f2d0
CC: @mdo

[Fix Issue #15701] Tooltip/popover destroy docs don't explain the delegation case.

Add to popovers

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Surprisingly those weren't reported by the validator; only in the grunt-contrib-htmlmin branch they were reported, so after digging into it, I saw they were indeed errors...

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Fixes #15465

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shorter, neater, well supported