MINOR: [Docs] Refine docs for some compute functions (#44305)

### Rationale for this change

Some trivial inaccurate and missing part in compute functions.

### What changes are included in this PR?

Refined the inaccurate and missing part.

### Are these changes tested?

No need.

### Are there any user-facing changes?

None.

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

docs/source/cpp/compute.rst

@@ -1707,7 +1707,7 @@ These functions select and return a subset of their input.
 +---------------+--------+--------------+--------------+--------------+-------------------------+-----------+
 | array_take | Binary | Any | Integer | Input type 1 | :struct:`TakeOptions` | \(3) |
 +---------------+--------+--------------+--------------+--------------+-------------------------+-----------+
-| drop_null | Unary | Any | - | Input type 1 | | \(1) |
+| drop_null | Unary | Any |   | Input type 1 | | \(1) |
 +---------------+--------+--------------+--------------+--------------+-------------------------+-----------+
 | filter | Binary | Any | Boolean | Input type 1 | :struct:`FilterOptions` | \(2) |
 +---------------+--------+--------------+--------------+--------------+-------------------------+-----------+
@@ -1851,15 +1851,18 @@ Structural transforms
  index *n* and the type code at index *n* is 2.
  * The indices ``2`` and ``7`` are invalid.
 
+Replace functions
+~~~~~~~~~~~~~~~~~
+
 These functions create a copy of the first input with some elements
 replaced, based on the remaining inputs.
 
 +--------------------------+------------+-----------------------+--------------+--------------+--------------+-------+
 | Function name | Arity | Input type 1 | Input type 2 | Input type 3 | Output type | Notes |
 +==========================+============+=======================+==============+==============+==============+=======+
-| fill_null_backward | Unary | Fixed-width or binary | N/A | N/A  | N/A   | \(1) |
+| fill_null_backward | Unary | Fixed-width or binary |  |   | Input type 1 | \(1) |
 +--------------------------+------------+-----------------------+--------------+--------------+--------------+-------+
-| fill_null_forward | Unary | Fixed-width or binary | N/A | N/A  | N/A   | \(1) |
+| fill_null_forward | Unary | Fixed-width or binary |  |   | Input type 1 | \(1) |
 +--------------------------+------------+-----------------------+--------------+--------------+--------------+-------+
 | replace_with_mask | Ternary | Fixed-width or binary | Boolean | Input type 1 | Input type 1 | \(2) |
 +--------------------------+------------+-----------------------+--------------+--------------+--------------+-------+
@@ -1872,7 +1875,7 @@ replaced, based on the remaining inputs.
  Also see: :ref:`if_else <cpp-compute-scalar-selections>`.
 
 Pairwise functions
-~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~
 Pairwise functions are unary vector functions that perform a binary operation on
 a pair of elements in the input array, typically on adjacent elements. The n-th
 output is computed by applying the binary operation to the n-th and (n-p)-th inputs,