BUG: Bug in downcasting inference with empty arrays (GH6733)
Merge pull request #6737 from jreback/infer

BUG: Bug in downcasting inference with empty arrays (GH6733)
DOC: small doc fixes (build warning in sym_diff + formatting in release notes)
Merge pull request #6739 from jorisvandenbossche/doc-fixes

DOC: small doc fixes (build warning in sym_diff + formatting in release notes)
added nunique and value_counts functions to Index