docs(core): add the description of version parameter for operator

[meteorgan]

Which issue does this PR close?

Part of #2611.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[meteorgan]

why is there no # before use opendal::Operator; at line 127 ? is a mistake or intentional ? do we have any guidelines for this ?

opendal/core/src/types/operator/operator.rs

Lines 122 to 133 in d479ac3

	/// # Examples
	///
	/// ```
	/// # use std::sync::Arc;
	/// # use anyhow::Result;
	/// use opendal::Operator;
	///
	/// # async fn test(op: Operator) -> Result<()> {
	/// let info = op.info();
	/// # Ok(())
	/// # }
	/// ```

[Xuanwo]

why is there no # before use opendal::Operator; at line 127 ? is a mistake or intentional ? do we have any guidelines for this ?

opendal/core/src/types/operator/operator.rs

Lines 122 to 133 in d479ac3

	/// # Examples
	///
	/// ```
	/// # use std::sync::Arc;
	/// # use anyhow::Result;
	/// use opendal::Operator;
	///
	/// # async fn test(op: Operator) -> Result<()> {
	/// let info = op.info();
	/// # Ok(())
	/// # }
	/// ```

I believe it's intentional. We don't have guidelines for this. Please feel free to write in your preferred way.

[Xuanwo]

Thank you @meteorgan for this!