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

[meteorgan]

Which issue does this PR close?

Part of https://github.com/apache/opendal/issues/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 ?

https://github.com/apache/opendal/blob/d479ac39efa9a88e10448ad81e47292b3b4ed688/core/src/types/operator/operator.rs#L122-L133

[Xuanwo]

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

https://github.com/apache/opendal/blob/d479ac39efa9a88e10448ad81e47292b3b4ed688/core/src/types/operator/operator.rs#L122-L133

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