[3.14] gh-138952: Document platform.machine() output casing inconsistency across platforms (GH-138962) (#139045)

gh-138952:  Document platform.machine() output casing inconsistency across platforms (GH-138962)
(cherry picked from commit dd15a2e11ea5d16ad42050a2723dec0e400ea394)

Co-authored-by: Aniket <148300120+Aniketsy@users.noreply.github.com>
Co-authored-by: AN Long <aisk@users.noreply.github.com>