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.

Also extend documentation of 'missing-reference'.