add descriptions for some code generator settings

[xuwei-k]

• generator.returnCollectionType
• generator.view
• generator.tableNamesToSkip
• generator.baseTypes
• generator.companionBaseTypes

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• https://github.com/scalikejdbc/scalikejdbc/blob/3.2.1/scalikejdbc-mapper-generator/src/main/scala/scalikejdbc/mapper/ScalikejdbcPlugin.scala#L98-L102
• https://github.com/scalikejdbc/scalikejdbc.github.io/blob/10ac86b65b757c/source/documentation/setup.html.md.erb#L105-L120