Add @Private attribute when 0.10 is released

[natke]

I removed this section from the style guide, as the @Private attributes does exist yet.

### Private or Internal Names ###

In many cases, a name is intended strictly for use internal to a library or project, and is not a guaranteed part of the API offered by a library.
It is helpful to clearly indicate that this is the case when naming functions and operations so that accidental dependencies on internal-only code are made obvious.
If an operation or function is not intended for direct use, but rather should be used by a matching callable which acts by partial application, consider using a name starting with `_` for the callable that is partially applied, and marking the function or operation as private with the <xref:microsoft.quantum.core.private> attribute:

```Q#
@Private()
operation _ApplyDecomposedOperation(decomposition : Int[], targets : Qubit[]) : Unit {
    // ...
}

I will add it back in once 0.10 is release