X-Ref #13969

Fix #16632

to include a note about `role="document"`

Initial fix to generalise use of modals. Generally, true role="dialog"
containers would require only forms/controls as content, with all other
text correctly and explicitly associated via
aria-labelledby/aria-describedby and similar...but as authors use modals
also as generic overlays, this extra role is required to get AT
(particularly NVDA) out of "forms" mode and reenable reading keys

as role="menu" is a very specific (and strict) ARIA pattern for
desktop-like application menus, and our dropdowns are often used
as pure navigation dropdowns, this change abandons ARIA menus for
a more open-ended and light-weight approach
(see http://heydonworks.com/practical_aria_examples/#submenus and
http://www.w3.org/WAI/tutorials/menus/flyout/#improve-screen-reader-support-using-wai-aria)
note that in dropdown.js, switched to now target ``.dropdown-menu``
instead of ``role["menu"]`` - this also prevents bootstrap scripts
from "bleeding" into non-bootstrap components on the same page.
also removed the ``role=["listbox"]`` part, which appears to be
vestigial/unused (only place in bootstrap that uses that
role are carousels, and their key handling is done separately)

[Close #16441] ScrollSpy requires non-body elements to have height and padding

http://www.w3.org/TR/wai-aria/roles#separator

[Close #16502] Clarify the bs-callout on multiple open modals

Currently it states overlapping modals, this isn't the only case as this truly applies to any instance when a user would want to open multiple modals at the same time. For this reason, I have modified the title of the callout to make this more clear.

Fixes Bootlint warnings:
	W007 Found one or more `<button>`s missing a `type` attribute.

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Anchor.js 1.0.0 drops the need to use anchor.css, so we remove it
from the codebase and move any leftover styles over to docs.css.

Closes #16488 by merging it.

Fixes #16470
Closes #16483

For uniformity, as we don't seem to use this "Enter <X>" phrasing anywhere else.

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Closes #16362.
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Refs https://github.com/twbs/bootstrap/issues/16362#issuecomment-96268469
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Closes #16122
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Fixes #16361
Refs #16356
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Mostly a duplication of the *Linked items* section, but for buttons.
Includes a note not to use the standard btn classes.

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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