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.

[skip sauce]
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

[skip sauce]
[skip validator]
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