DOC: point out the limitation of precision while doing serialization
- [x] closes #38437

Updated docstring for `to_json`, pointed out that the maximal possible value for `double_precision` is equal 15.

Hi @phofl. I made a small documentation improvement: pointed out in `to_json` docstring, that the maximal possible value for the `double_precision` is `15`. Could you please take a look at my pr?