mention possible definition of unique symbol in .h

I find it much more convenient to define the PY_ARRAY_UNIQUE_SYMBOL in a
header file #included by all files of the extension (than to repeat its definition in each and every file).

(In case you find the diff strange; I just reordered the #defines in order to make it more obvious that the #define NO_IMPORT_ARRAY can be separated from the other two lines, if they should be moved into a header file.)