[Format] Add clarifications to Columnar.rst about the contents of "null" slots in Varbinary or List arrays #22821
Description
Per mailing list discussion
Reporter: Wes McKinney / @wesm
Assignee: Liya Fan / @liyafan82
PRs and other links:
Note: This issue was originally created as ARROW-6451. Please see the migration documentation for further details.