Closes #2946.

@TarantoolBot document
Title: fixed module reload
There was a bug in tarantool documentation:
https://tarantool.io/en/doc/1.7/book/box/
box_schema/#lua-function.box.schema.func.reload
Now it is allowed to reload all functions in loadable
module via one method. Legacy method including finction
name is forbidden.
box.schema.func.reload("utils")       -- ok since now
box.schema.func.reload("utils.func1") -- forbidden since now

Global reload is still unsupported because it seems
to be useless.
box.schema.func.reload()              -- invalid!