[C++] Fix documentation on Newly Added Methods on ExecPlan

### Describe the bug, including details regarding any error messages, version, and platform.

We see the following error in the CIs : https://github.com/apache/arrow/pull/33623#issuecomment-1381326714

```bash
/arrow/cpp/src/arrow/compute/exec/exec_plan.h:427: error: The following parameter of arrow::compute::DeclarationToTable(Declaration declaration, bool use_threads=true, MemoryPool *memory_pool=default_memory_pool(), FunctionRegistry *function_registry=NULLPTR) is not documented:
parameter 'declaration' (warning treated as error, aborting now)
```

### Component(s)

C++

Inspect cpp/src/arrow/compute/exec/exec_plan.h around line 427 and compare the DeclarationToTable documentation with its declaration parameter list. Ensure the declaration parameter is documented, then rerun the documentation check or CI that reported the warning and confirm it no longer treats this error as fatal.

Written by the indexing model from the issue text.