Update information about Breakpoints

I removed the image because it was misleading while there is no UI for breakpoints in D8. I also removed the link to the change record, because that is 2 years old and things changed since than. I quoted the text from D8 help instead of the change record.

I think it is still not clear (after reading the change record, the built-in help and the online documentation) what is the use case of breakpoints module. From Amsterdam, I know one use case, this session record could help to write more information into this section of the documentation: https://amsterdam2014.drupal.org/session/drupal-8-breakpoints-and-responsive-images – I'll watch it later.