[2pt] Document that is_multikey func opt may be passed without sub-table

[TarantoolBot]

Product: Tarantool
Since: 2.10.0-beta2
Root document:

• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/create_index/#creating-a-functional-index
• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_schema/func_create/

SME: @ locker

Details

It's not necessary anymore to wrap is_multikey in opts table when
creating a function:

box.schema.func.create('my_func', {is_multikey = true})

is equivalent to

box.schema.func.create('my_func', {opts = {is_multikey = true}})

which still works, but deprecated.

Pease update this documentation entry.

Requested by @locker in tarantool/tarantool@66f5368

ToDo

• Update the description in https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/create_index/#creating-a-functional-index
• Research if the is_multikey should be mentioned in the reference https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_schema/func_create/. If yes, add the info about the option.