"Das ist unhöflich" would just mean "its impolite". With "doch" added, it gives "impolite" the meaning that it is comprehendible. Its certainly impolite. Something like "u should be knowing its impolite... and still ur doing it? Bad Boy/Girl!" But its not translated directly...

Doch here is only a word to strengthen the tone. In German language, doch is a "Partikel".

as Erica said it is used for emphasis and i think it also means the opposite of what is said before.or am i wrong?(i m a deutsch learner,too!)

Doch is a word for emphasis