Fixes #15653 — Add comment which explains that how a responsive image should be centered

Redo of #15657 which I couldn't get to rebase properly after all the merges were done last night.

[Fix Issue #15683] `readonly` input should not have a `not-allowed` cursor. Instead, this pull request displays a standard cursor and only displays the not-allowed cursor on disabled items.

Revise docs wording to clarify cursor display

People normally center images using text-align. However, the `.img-responsive`
class changes the display type to block. As a result you can no longer align the 
image with text-align. You must instead do it with margin: auto. There was no 
note about this on the web page, and no mention about setting the display to
block. Users were left on a frustrating journey to figure out why they can't center
images anymore!

http://www.oxforddictionaries.com/words/less-or-fewer

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Refs #15410
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Fixes #15555
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Closes #15288

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to clarify/hint that a ``<label class="sr-only">`` is not the only way
to do it

...and remove the redundant paragraph for basic abbreviation, which
simply repeats what was just said above.

shorter, neater, well supported

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Refs #11410

as discssued in
https://github.com/twbs/bootstrap/issues/15117#issuecomment-63029993

- added ``scope="row"`` to row headers
- made sure row headers are actual ``<th>`` elements
- removed ``scope="col"`` where it was redundant
- simplified table examples with (to me) unnecessary
``rowspan``/``colspan`` (unless you really want complex tables, in which
case we would need full-on ``id`` and ``headers`` attributes to make
sure each table cell has an explicit association with the related header
cells - overkill, in my opinion)
- one table (in typography) left header-less, as it's more or less presentational (though its structure is still important, hence not adding ``role="presentation"``)

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Make sample code match live example
Move <label> out of .input-group into .form-group instead
Change field from Email to Username to make it less redundant, slightly more realistic
Add <label> for Username field for consistency
Fixes #15118

...to actually make it consistent with what I said in the glyphicon docs
section...

Closes #14951 by merging it.

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Closes #14202.

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