Update docs for SystemTime Windows implementation

Windows now uses `GetSystemTimePreciseAsFileTime` on versions of Windows that support it.

Author: ollie27

library/std/src/time.rs

@@ -159,14 +159,15 @@ pub struct Instant(time::Instant);
 /// | DARWIN    | [gettimeofday]                                                       |
 /// | VXWorks   | [clock_gettime (Realtime Clock)]                                     |
 /// | WASI      | [__wasi_clock_time_get (Realtime Clock)]                             |
-/// | Windows   | [GetSystemTimeAsFileTime]                                            |
+/// | Windows   | [GetSystemTimePreciseAsFileTime] / [GetSystemTimeAsFileTime]         |
 ///
 /// [clock_time_get (Realtime Clock)]: https://nuxi.nl/cloudabi/#clock_time_get
 /// [`insecure_time` usercall]: https://edp.fortanix.com/docs/api/fortanix_sgx_abi/struct.Usercalls.html#method.insecure_time
 /// [timekeeping in SGX]: https://edp.fortanix.com/docs/concepts/rust-std/#codestdtimecode
 /// [gettimeofday]: http://man7.org/linux/man-pages/man2/gettimeofday.2.html
 /// [clock_gettime (Realtime Clock)]: https://linux.die.net/man/3/clock_gettime
 /// [__wasi_clock_time_get (Realtime Clock)]: https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs.md#clock_time_get
+/// [GetSystemTimePreciseAsFileTime]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemtimepreciseasfiletime
 /// [GetSystemTimeAsFileTime]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemtimeasfiletime
 ///
 /// **Disclaimer:** These system calls might change over time.