ARROW-13835: [Doc][Python] Add documentation for unify_schemas
ARROW-13835: [Doc][Python] Add documentation for unify_schemas

Closes #11057 from amol-/ARROW-13835

Authored-by: Alessandro Molina <15e43f79e4563fde6c3452592b20e2241738524d@turbogears.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

wesm/arrow,apache/arrow,wesm/arrow,wesm/arrow,icexelloss/arrow,kou/arrow,laurentgo/arrow,icexelloss/arrow,wesm/arrow,apache/arrow,kou/arrow,apache/arrow,kou/arrow,icexelloss/arrow,laurentgo/arrow,laurentgo/arrow,apache/arrow,wesm/arrow,wesm/arrow,laurentgo/arrow,kou/arrow,kou/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,kou/arrow,wesm/arrow,wesm/arrow,laurentgo/arrow,wesm/arrow,icexelloss/arrow,wesm/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,kou/arrow,apache/arrow,laurentgo/arrow,wesm/arrow,wesm/arrow,laurentgo/arrow,kou/arrow,kou/arrow,kou/arrow,icexelloss/arrow,icexelloss/arrow,icexelloss/arrow,apache/arrow,kou/arrow,laurentgo/arrow,wesm/arrow,icexelloss/arrow,apache/arrow,apache/arrow,icexelloss/arrow,icexelloss/arrow,wesm/arrow,laurentgo/arrow,kou/arrow,kou/arrow,apache/arrow,apache/arrow,apache/arrow,laurentgo/arrow,apache/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,kou/arrow,laurentgo/arrow,apache/arrow