Fix up FileProcessor.file_tokens property

We opted to not copy the file_tokens attribute each time it's accessed
in the merge request discussion but it was never reflected in the code.
Further, the attribute had no documentation or docstring, so we've added
that. Finally, we address a personal style nit that I otherwise wouldn't
have picked at.

src/flake8/processor.py

@@ -103,15 +103,22 @@ class FileProcessor(object):
 
     @property
     def file_tokens(self):
+        """The complete set of tokens for a file.
+
+        Accessing this attribute *may* raise an InvalidSyntax exception.
+
+        :raises: flake8.exceptions.InvalidSyntax
+        """
         if self._file_tokens is None:
             line_iter = iter(self.lines)
             try:
                 self._file_tokens = list(tokenize.generate_tokens(
-                    lambda: next(line_iter)))
+                    lambda: next(line_iter)
+                ))
             except tokenize.TokenError as exc:
                 raise exceptions.InvalidSyntax(exc.message, exception=exc)
 
-        return self._file_tokens[:]
+        return self._file_tokens
 
     @contextlib.contextmanager
     def inside_multiline(self, line_number):