[improve][doc]Add introduction of transaction isolation level in the txn-advanced-features.md

[hzh0425]

This PR adds doc for apache/pulsar#21114

This PR fixes #xyz

• doc
• doc-required
• doc-not-needed
• doc-complete

[tisonkun]

We should not merge this patch until PIP-298 is accepted and merged. Is it correct?

[visortelle]

The PIP-298 has been merged: apache/pulsar#21114

I applied slight changes that shouldn't affect sense much:
384084d...2b4c747#diff-eefcce381353ec170e6087b3c59f413c4e3f6081f93de753dd2714d725bebdbd

Merging.

[shibd]

@visortelle Please make sure someone approves and all concerns are addressed before merging the PR.

[tisonkun]

@shibd Who is "someone"? I'd prefer you name the list of component experts so that @visortelle can ping for review.

Note that @visortelle is a committer and we don't have class among committers but if you know experts that should review changes on these components, we can collaborate.

[shibd]

I just want to remind @visortelle that PRs should be reviewed by someone before they can be merged, and we should also take comments into consideration.

[visortelle]

@shibd ok, no problem. Let me know if something is wrong with this specific PR.

[shibd]

Thanks, This PR should not be merged because it has not been implemented yet. Please refer to this comment: #712 (comment)

[visortelle]

I agree, it is my fault.

I think it makes sense to:

• create a PR with the revert commit
• create a new draft PR with the same changes

@shibd WDYT?

[visortelle]

@shibd

• reverted changes 570f77e
• new draft PR [improve][doc]Add introduction of transaction isolation level in the txn-advanced-features.md  #906