Document relminmxid and datminmxid

I introduced these new fields in 0ac5ad5134f27 but neglected to add them
to the system catalogs section of the docs.

Per Thom Brown in message
CAA-aLv7UiO=Whiq3MVbsEqSyQRthuX8Tb_RLyBuQt0KQBp=6EQ@mail.gmail.com
Update pg_resetxlog's documentation on multixacts

I added some more functionality to it in 0ac5ad5134f27 but neglected to
add it to the docs.

Per Peter Eisentraut in message
1367112171.32604.4.camel@vanquo.pezone.net