[C++] [Documentation] add discussion of Union.typeIds to Layout.rst

[asfimport]

Union.typeIds is poorly documented and the corresponding property in UnionType is confusingly named type_codes. In particular, Layout.rst doesn't include an explanation of Union.typeIds and implies that an element of a union array's type_ids buffer is always the index of a child array.

Reporter: Ben Kietzman / @bkietz

_{Note: This issue was originally created as ARROW-5933. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.