[Improvement] Document that only SQL/TRINO functions are exposed through the Trino connector

### What would you like to be improved?

A function registered with any language/runtime other than SQL/TRINO (e.g. a Python or Java implementation with runtime SPARK) is silently absent from the Trino connector: it does not appear in `SHOW FUNCTIONS`, and calling it returns a bare `Function ".." not registered` error. Nothing in the product tells the user the function was filtered rather than missing.

The filter in `GravitinoMetadata.isTrinoSqlImplementation` is intentional; the problem is that it is invisible.

### How should we improve?

- State the boundary in the Trino connector docs and the function docs: only functions with language `SQL` and runtime `TRINO` are visible to and callable from Trino; other implementations are managed in Gravitino but not exposed through this connector.
- Show per implementation in the web UI function detail view whether it is exposed through the Trino connector.

A follow-up could surface non-Trino functions and fail invocation with a message naming the registered runtime, but that is connector work with its own risk.

Start with GravitinoMetadata.isTrinoSqlImplementation and locate the Trino connector docs, function docs, and web UI function detail view. Document that only SQL/TRINO implementations are exposed, and make the UI indicate exposure per implementation. Confirm the documentation and UI clearly distinguish managed non-Trino implementations from callable Trino functions.

Written by the indexing model from the issue text.