Update docs for expression defaults

[dveeden]

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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