Document that multiple rustpkg processes are a bad idea

[catamorphism]

rustpkg doesn't lock the workcache database file at all, so it's a bad idea to run multiple instances of it. Document this -- particularly in the documentation on "how to use make with rustpkg", so people know not to use make -j with rustpkg.

Also see #9649

[flaper87]

Closing rustpkg is deprecated