implement multiple combine statement
add docblocks
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