index entries for library declarations should point at the declaration itself, not the start of the synopsis

This is particularly obvious for very long synopses. We generally write index entries at the start of the synopsis block, but we should really be pointing them at the point within the synopsis where the indexed entity is declared.

We probably need a new macro for marking up a library entity in a synopsis that adds an index entry and emits the name of that entity, so we can write something like:

int @\libdecl{myfunction}@(int, int);

to emit and index the name (with another form for member functions).

The issue names no file, test, or entry point, so first locate the synopsis markup and existing index-entry macros in the draft sources. Compare library entities and member functions, then verify that the proposed markup emits the entity name and places its index target at the declaration rather than the synopsis start.