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MINOR: [Docs] Add fill_null functions to docs

Add the docs for the functions from ARROW-1699/PR #11853 since they were omitted in the PR.

Closes #12082 from lidavidm/arrow-1699

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>