DOC initialize string with bytes

As-is the example doesn't compile:

```
Error compiling Cython file:
------------------------------------------------------------
...
        cdef string s = string('abcdefg')
                             ^
------------------------------------------------------------

my.pyx:125:30: no suitable method found
```

I'm using Python 3.4, Cython 0.21 and the following options: c_string_type=str, c_string_encoding=ascii