[2pt] New way to define index parts

[mary3000]

New way to define index parts

Product: Tarantool
Since: 2.7.1
Audience/target:
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space
SME: @mary3000
Peer reviewer: @

Documentation needs to be updated according to tarantool/tarantool#2866:

Previously accepted formats of index parts:

parts = {field1, type1, field2, type2}
-- or
parts = {{field1, type1, ...}, {field2, type2, ...}}

Now it is allowed to write without extra brace if there is one part only:

parts = {field1, type1, ...}