do getRawState() same as in other objects
do $specifications same as in Select object
add alignColumns() and fix getRawState()
#5142 - cleaning up docblocks
#5142 - turned `protected` members into privates, as the class has standalone functionality and is based on `AbstractSql`
#5142 - fixed static analysis warnings
#5142 - adding missing docblock to `Zend\Db\Sql\SqlInterface`
#5142 - PSR-2 compliance of `Zend\Db\Sql\PreparableSqlInterface`
#5142 - Expecting exceptions when passing invalid select statements to `Zend\Db\Sql\Combine#combine()`
#5142 - Cleaning up static introspection analysis results in `CombineTest` implementation
#5142 - Reverting inherited property visibility, more detail in exception messages, reverting `void` method return value (non-void)
Merge branch 'feature/#5142-allow-multiple-combined-union-statements' into develop

Close #5142
allow use different types of arguments in any position
add docblock and comments
fix comment
#5320 - turn protected members into privates before merge
Revert "#5320 - turn protected members into privates before merge"

This reverts commit a00d1c45445f2d2347b29bb0779346222c507e07.
#5320 - removed unused protected method, cleaned up infinite if/else chains that should never be merged
#5320 - adding missing docblock
#5320 - cs fixes in the `Operator` predicate