It's funny people don't do this as much with other languages. For example, nobody describes pure Javascript as "js/html" even though you can have lots of js inside HTML code.
C is actually part of of the C++ standard. Within the ISO C++11 standard there is the C99 standard with some adjustments and exclusions. The integration is quite tight. So one can say C/C++ with some clarity.
Library writers often go thorough a lot of work to write extensive object oriented interfaces around C libraries. And they intend that the user use the C++ version not just import the C version in there C++ code. So in that case it is a necessity to use C/C++ because there are two separate interfaces.
In the general case I would guess that 'most' C programmers write some C++ and vise versa. So you have a very large overlap of C/C++ programmers. Objective C on the other hand is probably a significantly smaller group and is simply not big enough for C/C++ communities to think to much about.
That's not what I've seen. People usually either write C++ exclusively or C exclusively and hate the other language (probably more C writers that hate C++ than C++ writers that hate C).
Some people write very C-accented C++, with mallocs and frees instead of news and deletes and so forth (these are usually people who prefer C over C++ but are writing C++ for whatever reason). I guess those people could be said to be writing "C/C++". But in general I think people overestimate how close the two language communities are. What I have seen is that people like one language and deride the other.
so at the bare minimum C/C++ will possibly increase your user base.
(edit)
You should not assume that C++ programmers are going to wade through your library code and rewrite your library with the proper signatures. You as the library writer should have done that and tested it, then advertised to prospective users that your library is ready for use with C++.