Method signatures only show the first defined return type about yard

[lsegal]

Show multiple types in signature return types and add heuristics for nullable/splat returns

Summary of the new heuristics are as follows:

• Single return type: (Type)
• Two return types: (TypeA, TypeB)
• More than two: (TypeA, ...)
• @return [Type, nil]: (Type?) [? is superscripted]
• @return [Type, Array]: (Type+) [+ is superscripted]
• @return [void]: no return type shown

Closed by 497696a