Add comment for clusters and UI display

[BlackBear2003]

What's the purpose of this PR

Add comment for clusters and UI display

Which issue(s) this PR fixes:

Fixes #https://github.com/apolloconfig/apollo/issues/5067

Brief changelog

it looks like:

Follow this checklist to help us incorporate your contribution quickly and easily:

• [x] Read the Contributing Guide before making this pull request.
• [x] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• [ ] Write necessary unit tests to verify the code.
• [ ] Run mvn clean test to make sure this pull request doesn't break anything.
• [ ] Update the CHANGES log.

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (6657a58) 49.65% compared to head (8db44fa) 49.60%.

Files	Patch %	Lines
...com/ctrip/framework/apollo/biz/entity/Cluster.java	0.00%	4 Missing :warning:
.../ctrip/framework/apollo/common/dto/ClusterDTO.java	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5072 +/- ## ============================================ - Coverage 49.65% 49.60% -0.06% + Complexity 1905 1904 -1 ============================================ Files 372 372 Lines 11558 11564 +6 Branches 1127 1127 ============================================ - Hits 5739 5736 -3 - Misses 5478 5488 +10 + Partials 341 340 -1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[nobodyiam]

This looks great! Please find some comments below:

1. I think the comment is similar to the remarks of namespace, so it's better to use the same term, e.g. Remarks and 备注

2. I think the comment should not be mandatory, as it's not a necessary information

3. When displaying the comment, it's not necessary to display the prefix comment:

[BlackBear2003]

I think the comment is similar to the remarks of namespace, so it's better to use the same term, e.g. Remarks and 备注

changed terms at front-end from 'comment' to 'remarks'.

I'm not sure should we change the sql and entity's field name? I see there is comment in AppNamespace, Commit and Item while there is not remarks. So I just use remark when UI display.