DOC Adds details about the pandas choice

Author: thomasjpfan

slep018/proposal.rst

@@ -115,8 +115,10 @@ Discussion
 A list of issues discussing Pandas output are: `#14315
 <https://github.com/scikit-learn/scikit-learn/pull/14315>`__, `#20100
 <https://github.com/scikit-learn/scikit-learn/pull/20100>`__, and `#23001
-<https://github.com/scikit-learn/scikit-learn/issueas/23001>`__.
-
+<https://github.com/scikit-learn/scikit-learn/issueas/23001>`__. This SLEP
+proposes configuring the output to be pandas because it is the DataFrame library
+that is most widely used and requested by users. The ``set_output`` can be
+extended to support support additional DataFrame libraries in the future.
 
 References and Footnotes
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