Remove examples in doc-comments of AsciiExt methods

The doc comments were incorrect before: since the inherent ascii methods
shadow the `AsciiExt` methods, the examples didn't use the `AsciiExt` at
all. Since the trait will be deprecated soon anyway, the easiest solution
was to remove the examples and already mention that the methods will be
deprecated in the near future.

Author: LukasKalbertodt