Warn about IE emulation modes in docs+examples

[cvrebert]

This script, which I've pretty exhaustively tested manually using Sauce, alerts the user about the perils of trying to use IE's emulation modes to test older versions of IE.
These emulation modes can behave differently from actual IE, which can lead users to file Bootstrap bugs about supposed IE8 problems which don't occur when tested properly using actual IE8.

Examples of emulation inaccuracy:

• Navbar is not working properly in IE8 #13177
• http://stackoverflow.com/questions/5612170/web-testing-for-ie-how-accurate-is-ietester
• http://stackoverflow.com/questions/19446584/why-doesnt-ie11-honour-conditional-comments-even-when-emulating-ie8-document-mo
• http://stackoverflow.com/questions/10249454/how-well-does-ie7-8-mode-in-ie9-compare-to-actually-running-ie7-8

This should hopefully reduce the amount of time wasted chasing down false IE8 bug reports.
/cc @twbs/team for review

[XhmikosR]

I personally find it very useful to try things locally without much hassle. But I agree, people shouldn't use it especially with IE11 not respecting conditionals anymore.

I'll try the patch in action later and let you know.

[cvrebert]

I tested every actualIE-emulatedIE combo for IE 8-11 via Sauce and it worked, so it should be fine technically speaking.
@mdo Does the concept+wording seem good to you?

[XhmikosR]

BTW, I wouldn't remove semicolons... Sure it's not consistent with the current style, but there's no real progress on that, so I'd say use what you prefer in that regard.

[mdo]

Hmm, do we need to have this in our code or can we not just be more specific in our bug report docs? I know there will inevitably be overhead, but this only addresses the docs and our examples. Any js bin reports or other demos will still face similar problems.

[cvrebert]

can we not just be more specific in our bug report docs?

Very few people read the bug report docs.

this only addresses the docs and our examples. Any js bin reports or other demos will still face similar problems.

• A significant portion of the relevant issues are about our docs or examples supposedly not working.
• Lots of folks probably (in the same browser) at least consult or in some cases crosscheck against our docs before they get to the point of deciding that they need to file an issue. This new warning will also cover those cases.
• Every little bit helps when you get as many issues as we do. 😄

Are you okay with the phrasing of the warning message?

[cvrebert]

We can always further tweak the message later.