[2pt] tuple: allow nullable multikey index root

[PersDep]

Update the documentation for space_object:create_index() to reflect
that it is now safe for multikey index root to be nullable.
Previously it was not safe to insert, for example, box.NULL as an array
field which elements are the part of multikey index due to error in
counter of multikey index keys in tuple. It was partly fixed using
default non-nullability for tuple fields. In 2.3.3 the restriction on
nullable multikey index root was introduced. Now the counter problem is
fixed and all tuple fields are nullable by default as before 2.2.1.

Requested by @PersDep in tarantool/tarantool@bfeb61b.