Not all headers are indexed in the main index.

The main index (not the library names index) only has index entries for the C library headers and some freestanding ones such as `<exception>`, `<initializer_list>` and `<new>`.

It has entries for headers that aren't even part of C++, such as `<stdatomic.h>` and <threads.h>`but not  anything like`<vector>`.

Is that intentional?
