update explanatory comment about iOS date/time inputs bug

Per:
* http://jsbin.com/purer/2/edit
* https://github.com/cvrebert/bs-css-hacks/blob/6e8aa78384a005b007fc45327c9516dcf6ec0f30/README.md#ios-temporal-input-line-height
Thanks @mdo <3

[skip sauce] [skip validator]
Merge pull request #13651 from twbs/ios-temporal

update explanatory comment about iOS date/time inputs bug