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

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

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