Fix #9944 (LaTeX writer visit_desc_content())

[jfbu]

Relates #9926

With this, the following

.. option:: -mmmx
            -msse
            -msse2

   .. note::

      A note in the option description

.. c:function:: PyObject *PyType_GenericAlloc(PyTypeObject *type, Py_ssize_t nitems)

   .. note::

      Extra vertical space also in this case if note is first item

.. cpp:class:: Data

   .. cpp:union:: @data

      .. cpp:var:: int a

      .. cpp:var:: double b

Explicit ref: :cpp:var:Data::@data::a. Short-hand ref: :cpp:var:Data::a.

produces

Comparing to the output shown in issue #9944, it only fixes the vertical space for the option with note and nested declaration case, not for the note at start of function description.

This PR removes from LaTeX markup an ~ which was added (14 years ago) at 3761223 to fix some "bad formatting issue in case of empty description".

(edit after further investigation) the ~ was needed at least in nested cases like the cpp example above. Without it and at Sphinx <1.5 the cpp example above gives mangled pdf output.

But since 1.5, the LaTeX mark-up adds \pysigstartmultiline/\pysigstopmultiline pairs. The \pysigstopmultiline appears to fix the same issue but without the extra spacing. So it seems ~ should not be inserted, at least in such examples.

note: I did not need to update tests as apparently no unit test existed to check for the ~ insertion in latex object descriptions in some cases.

this PR replaces #9945 which was accidentally closed from renaming the branch at my fork underlying it

[tk0miya]

LGTM. But it's better to merge this into the 4.x branch instead.

[jfbu]

Thanks for review @tk0miya I have rebased on 4.x and will merge after testing completes