Document that os.cpus() returns times in milliseconds, not ticks

This is purely a documentation change, and corresponds to a pull request for libuv I sent a few minutes ago: most platforms are already doing this, the pull request changes the linux and solaris ports (I don't know what the AIX port does).

Note this is a change to the documentation of an API marked "frozen": however it changes the documentation to be closer to what actually happens.  However it won't be strictly true until the libuv code is merged into node proper.