Update docs for SystemTime Windows implementation
Update docs for SystemTime Windows implementation

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

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust