Bug 16957 add a health warning about floating date/time usage

Related to i18n bug:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=16957

(cherry picked from commit 6704d9ac2cf55a8eae2b0e12f675d74df4c3d65f)
[Bug 16960] Removing input type="datetime-local"

Removing the definition of input type="datetime-local" and its sections
and references. Also updated one of the example explanitory text. See
also:
http://lists.w3.org/Archives/Public/public-html/2014Feb/0003.html

(cherry picked from commit 3e5df1ee1aebed37571a23c9b62adc74a92a04b9)