DOC: Clarify behavior for Series with dict-like data and index
- [x] closes #34534


I think the previous description was wrong. We only reindex if the index is not None, not the opposite.

Added the example.

I think we should backport here? The sentence I fixed was added in 1.2 and is wrong > Added the example.
> 
> I think we should backport here? The sentence I fixed was added in 1.2 and is wrong
yup can backport thanks @phofl  @meeseeksdev backport 1.2.x