Fix some comments in dwarf2/cooked-index.h

This fixes a couple of comments in dwarf2/cooked-index.h.

The comment by cooked_index_entry::canonical mentions C++, but this
field can also be different from 'name' in other situations.  Rather
than enumerate the cases here (which doesn't seem important), make the
text a little less specific.

Also, cooked_index_entry::write_scope doesn't document its "for_main"
parameter -- and it is misnamed in the prototype as well.

Reviewed-By: Tom de Vries <tdevries@suse.de>