[docs](query plan) Add a description `explain graph` and `desc graph`

[iceqing]

Proposed changes

Issue Number: close #xxx

Problem summary

Add description for explain graph and desc graph commands to avoid inconsistent documentation.

为explain graph 与 desc graph命令添加描述，避免文档前后不一致。

Checklist(Required)

1. Does it affect the original behavior:
   • [ ] Yes
   • [x] No
   • [ ] I don't know
2. Has unit tests been added:
   • [ ] Yes
   • [ ] No
   • [x] No Need
3. Has document been added or modified:
   • [x] Yes
   • [ ] No
   • [ ] No Need
4. Does it need to update dependencies:
   • [ ] Yes
   • [x] No
5. Are there any changes that cannot be rolled back:
   • [ ] Yes (If Yes, please explain WHY)
   • [x] No

Further comments

If this is a relatively large or complex change, kick off the discussion at [email protected] by explaining why you chose the solution you did and what alternatives you considered, etc...