[PR] Minor: Add upgrade guide for `Expr::WindowFunction` [datafusion]

[via GitHub]

alamb opened a new pull request, #16313:
URL: https://github.com/apache/datafusion/pull/16313

   ## Which issue does this PR close?
   
   - Part of https://github.com/apache/datafusion/issues/15771
   - Related to https://github.com/apache/datafusion/pull/16207
   
   ## Rationale for this change
   
   https://github.com/apache/datafusion/pull/16207 is a breaking API change and 
it would be nice to help people when upgrading
   
   
   ## What changes are included in this PR?
   
   Add a section in the upgrade guide about upgrading their code
   
   Here is how it looks rendered:
   
   ![Screenshot 2025-06-06 at 4 15 36 
PM](https://github.com/user-attachments/assets/1c4b7555-9ac0-4e7b-8269-f255644b07b2)
   
   ## Are these changes tested?
   CI 
   ## Are there any user-facing changes?
   
   Just docs, no code change


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