In #7487, SingledispatchFunctionDocumenter is merged into
FunctionDocumenter.  SingledispatchMethodDocumenter is also.  As a result,
They are no longer needed.  So this deprecates them.

This postpones the evaluation of LaTeX packages via
``app.add_latex_package()`` to just before writing .tex file.  That
allows extensions to add LaTeX packages during reading and resolving
phase.

Add a special class `_Empty` to make the comparison of
:exclude-members: option simply.  It never matches to any members.

Now :private-member: option can take an explicit list of member names to
be documented as its arguments (like :special-members: does).

The texinfo writer remembers the current desc node to render
a desc_annotation node.  This makes the mechanism robust to support
nested object descriptions.