`SHOW FUNCTIONS` allowed syntax does not match source/intent

SHOW FUNCTIONS
https://github.com/sqlparser-rs/sqlparser-rs/blob/7282ce22f9f031a2c9fd5831427f01f2f0dbb978/src/ast/mod.rs#L2450-L2453

it allow syntax
```
SHOW FUNCTIONS
SHOW FUNCTIONS LIKE 'pattern'
SHOW FUNCTIONS ILIKE 'pattern'
SHOW FUNCTIONS WHERE
```

however, it's also **documented** to match Presto's SHOW FUNCTIONS
which doesn't allow ILIKE nor WHERE (and wouldn't know how the WHERE should be applied)
https://prestodb.io/docs/current/sql/show-functions.html

worth noting that Trino's SHOW FUNCTIONS is similar, but allows additional schema filter
https://trino.io/docs/current/sql/show-functions.html

从 src/ast/mod.rs 的第 2450-2453 行开始，将文档中记录的 SHOW FUNCTIONS 语法与链接的 Presto 和 Trino 参考进行比较。确定此 parser 旨在支持哪种方言行为，然后调整接受的语法及其文档，使受支持的形式保持一致。

由索引模型根据 Issue 内容生成。