Suppress -Wimplicit-fallthrough warning in new LIMIT WITH TIES code.

Suppress -Wimplicit-fallthrough warning in new LIMIT WITH TIES code.

The placement of the fall-through comment in this code appears not to
work to suppress the warning in recent gcc.  Move it to the bottom of
the case group, and add an assertion that we didn't get there through
some other code path.  Also improve wording of nearby comments.

Julien Rouhaud, comment hacking by me

Discussion: https://postgr.es/m/CAOBaU_aLdPGU5wCpaowNLF-Q8328iR7mj1yJAhMOVsdLwY+sdg@mail.gmail.com