Remove extraneous semicolon from uses of relptr_declare().

Remove extraneous semicolon from uses of relptr_declare().

If we're going to write a semicolon after calls of relptr_declare(),
then we don't need one inside the macro, and removing it suppresses
"empty declaration" warnings from pickier compilers (eg pademelon).

While at it, we might as well use relptr() inside relptr_declare(),
because otherwise that macro would likely go unused altogether.

Also improve the comment, which I for one found unclear,
and provide a specific example of intended usage.