[Edit] SQL Windows Function

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Add new content (definitions, codeblocks, etc.)

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We would like to edit the term entry Lag in the concept entry Window-Function under SQL. The entry lives in content/sql/concepts/window-functions/terms/lag/lag.md.

Please refer to the concept entry template, content standards, and markdown style guide when working on the PR for this issue.

Please:

1. Add more example with default value and use of partition by.
2. Add one sentence on the usecases.

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