Improve string forms and PyUnicode_Resize() documentation

Remove also the FIXME for resize_copy(): as discussed with Martin, copy the
string on resize if the string is not resizable is just fine.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 46332fb..cb4813f 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -536,7 +536,8 @@
             return NULL;
         }
         return copy;
-    } else {
+    }
+    else {
         PyUnicodeObject *w;
         assert(_PyUnicode_WSTR(unicode) != NULL);
         assert(_PyUnicode_DATA_ANY(unicode) == NULL);
@@ -1294,7 +1295,6 @@
     if (old_length == length)
         return 0;
 
-    /* FIXME: really create a new object? */
     if (!unicode_resizable(unicode)) {
         PyObject *copy = resize_copy(unicode, length);
         if (copy == NULL)