docs: eliminate note invalid option block errors

Using Sphinx 1.1.2, errors like the following are generated for
documentation/Reference.txt:

  /tmp/seq-head/documentation/Reference.txt:85: ERROR: Error in "note"
  directive:
  invalid option block.

  .. note:: :token:`line_number` may only contain decimal numbers, and
     :token:`file_name` must be a single string (no automatic string
     concatenation).

The error seems to occur when a text role (e.g. ":token:") is used in
the content. Adding a blank line between the directive and the content
eliminates the error.