[3.13] gh-127852: add remark about ',' separator (GH-127854) (#127941)

Specify that it is valid for floats and ints with 'd' presentation and an error otherwise.
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(cherry picked from commit e2325c9db0650fc06d909eb2b5930c0573f24f71)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>