Update docs for expression defaults (#19685) (#19896)

[ti-chi-bot]

This is an automated cherry-pick of #19896

This is an automated cherry-pick of #19685

What is changed, added or deleted? (Required)

Use https://github.com/pingcap/tidb/blob/5e8d1b8ae1de446841c105d0c177320c88ff893d/pkg/ddl/add_column.go#L628-L787 to update the docs

See also:

• ddl: support specifying expr rand() as column default value tidb#32608
• ddl: Allow JSON functions for defaults tidb#52786

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[hfxsd]

/approve

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