[VL] document the gap on unsupported case for complicated functions

### Description

For functions like `from_json/to_json` it's not fully supported and will use fallback code path under some configurations. Currently It's not covered by the automated function support check tool - the function support table may not be accurate(https://github.com/apache/incubator-gluten/blob/main/docs/velox-backend-scalar-function-support.md), better to add extra document on these cases

### Gluten version

None

Start with docs/velox-backend-scalar-function-support.md and inspect the from_json/to_json entries alongside the automated function support check described in the issue. Document the configurations where these functions use the fallback path and make the support information accurately reflect those cases.

Written by the indexing model from the issue text.