How to use floating-point storage for time data type
From the linked page you provide:


  Note: When timestamp values are stored as eight-byte integers (currently the default), microsecond precision is available over the full range of values. When timestamp values are stored as double precision floating-point numbers instead (a deprecated compile-time option), the effective limit of precision might be less than 6. ...


What this says is that in order for timestamp values to be stored as double precision floating-point (and to allow precision up to 10), you need to compile Postgres yourself with that option. 

It also says that the precision is then not exactly 10 but it varies depending on the value and may even be less than 6 for some (very high or very low) values.