Remove holder.js references from syntax-highlighted example HTML

Our use of Holder.js in the docs is a mere implementation detail.
Don't want to confuse people into thinking Holder.js
is somehow required by the Thumbnail component.

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Remove reference to old fixed WebKit bug in Less comment.

Per https://github.com/twbs/bootstrap/issues/1969#issuecomment-36971094

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Merge pull request #15427 from twbs/defunct-bug-1969

Remove reference to old fixed WebKit bug in Less comment.
Reference iOS Safari temporal input bug in explanatory Less comment

Refs #11266, #13098, #15415

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automatic grunt dist
add more refs to browser-related bugs in Less comments
IE11 & current Firefox are still affected.

Refs #11623
Add summary of #11526 to explanatory Less comment

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Merge pull request #15426 from twbs/hide-holder

Remove holder.js references from syntax-highlighted example HTML
Merge pull request #15428 from twbs/browser-bugs

add/update/summarize browser bug refs in Less comments
grunt
automatic grunt dist
Fixes #15422: Alternate fix for temporal input sizing in input groups
Merge branch 'master' of https://github.com/twbs/bootstrap

Conflicts:
	dist/css/bootstrap.css.map
grunt
bump remarkable to ~1.6.0
update shrinkwrap
Merge pull request #15451 from twbs/bump-remarkable

bump remarkable to ~1.6.0
bump load-grunt-tasks to ~2.0.0
update shrinkwrap

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