Function grant//create doc improvments

[bigbes]

1. Specifying if_not_exists=true would cause error: Function '...' already exists if the function already exists. in https://tarantool.org/doc/book/box/authentication.html?highlight=authentication#functions-and-the-func-space

true must be replaced with false

tarantool> box.schema.func.create('f1')

---
...
tarantool> box.schema.func.create('f1')

---
- error: Function 'f1' already exists
...
tarantool> box.schema.func.create('f1', {if_not_exists=true})

---
...

2. Mention, that if you want grant function execution, you need to create that function first here https://tarantool.org/doc/book/box/box_schema.html#lua-function.box.schema.user.grant