Functional index function doesn't have to sandboxed anymore

Product: Tarantool
Since: 3.2
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_schema/func_create/#lua-data.function_options.is_sandboxed
https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/create_index/#creating-a-functional-index

SME: @ locker

Details

The user may now create a functional index using a function that doesn't
have the flag is_sandboxed. Note that a functional index function must
be deterministic and isn't allowed to yield or access the database so
the flag is_sandboxed is still recommended. However, there are cases
when the user might want to clear the flag. For example, it's the only
way to let a functional index function to call a deterministic function
defined in a module (e.g. json.encode).
Requested by @locker in https://github.com/tarantool/tarantool/commit/9e616ab6f2f94ba2bcad3044187c474a1c05a8c5.

Start with the two linked root documents: box_schema/func_create and box_space/create_index. Review the is_sandboxed option and functional-index guidance, then update them to explain that sandboxing is not required, while the function must remain deterministic and must not yield or access the database. Done means the documentation also covers deterministic module functions such as json.encode.