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.

The PR removes from LaTeX markup an ~ which was added 14 years ago at 3761223. The ~ causes LaTeX to enter into paragraph mode and then this causes vertical space if some "display block" type of entity (such as a note) follows).

It seems not to be needed anymore. I could not find an example where the ~ to avoid empty contents would avoid a "formatting bug" and don't know where to look.

[jfbu]

(I had completely forgotten to check the unit tests, but in fact I see from succesful testing that none existed for a case where latex writer would output the ~ or at least none tested for its presence in output...)

[jfbu]

Sorry this got closed from me renaming my local branch via the web interface at my fork. Will open a new PR with the renamed branch.