Python doc is in function body in triple-quoted string. But it is still comment, so with help from Mirek Ruzicka, i wrote syn region to python.vim syntax file.
New lines in python.vim file
Write this lines after pythonRawString region definition to your python.vim file if you want colorize python doc string as comments.
syn region pythonComment
\ start=+\(:\n[\t ]*\)\@<=\z('''\|"""\)+ end=+\z1+ keepend
\ contains=pythonEscape,pythonTodo,@Spell