add distsql rdl alter and drop default single table rule doc.

[tianhao960]

Fixes #20989 .

Changes proposed in this pull request:

• add distsql rdl alter and drop default single table rule doc

---

Before committing this PR, I'm sure that I have checked the following options:

• My code follows the code of conduct of this project.
• I have self-reviewed the commit code.
• I have triggered maven check: mvn clean install -B -T2C -DskipTests -Dmaven.javadoc.skip=true -e.
• I have made corresponding changes to the documentation.
• I have added corresponding unit tests for my changes.