Document additional differences in SystemTime across platforms

[rylev]

In rust-lang/measureme#173 we discovered that our tests were failing on Windows because SystemTime on Windows (which is backed by FILETIME) only works in 100 nanosecond intervals, and when we were adding a Duration of less than 100 nanoseconds to a SystemTime we were getting back the same SystemTime.

While this isn't a bug as SystemTime makes it clear that it uses platform specific backing implementations, it would be nice if it were documented that some of the operations on SystemTime might perform differently on different platforms.