Improve documentation, warnings, and fix a few regressions

- Fix a regression where headings 4 would use the default browser font
- Document all the things
- Raise warnings when authors use deprecated mixins (without raising warnings coming from the module itself)
Because if a px value is passed (instead of an integer), Sass will still accept the argument.

It's usually a good think to be forgiveful in the typing of arguments passed. Here:

```scss
// Both will work:
@include oFtTypographyFontOffset(4px);
@include oFtTypographyFontOffset(4);
```