typo
Clean up mixins, fix #9100 (no ms-keyframes)

- Spacing tweaks in the general mixins
- Add comments to transform mixins about IE support
- Remove `-ms-keyframes` because only IE10+ supports it, and without
prefix
update modals demo code
more docs changes
Merge branch '3.0.0-wip' of github.com:twbs/bootstrap into 3.0.0-wip