[JANRAIN-1223] Document class for dirty form state

Adds message to documentation mentioning the use of `.has-warning` to
indicated unsaved changes.

This also changes the color of error message text from white back to red. @bmock - you mentioned that the white was a side effect of changes made to the $state-danger-text variable. But it made validation error messages on forms invisible on white or light colored background. Can you check that my changing it back to red doesn't mess up anything elsewhere?