Update rustpkg man page.

Closes #9221.

"rustpkg test" isn't implemented yet, so it shouldn't be in the manpage. Referring interested parties to the manual is probably
the right thing for now; eventually, these documents should merge.
auto merge of #9234 : steveklabnik/rust/rustpkg_manpage, r=cmr

Closes #9221.

"rustpkg test" isn't implemented yet, so it shouldn't be in the manpage. Referring interested parties to the manual is probably
the right thing for now; eventually, these documents should merge.

/cc @catamorphism