Documentation: union_relations uses "union all"

[owenprough-sift]

Describe the feature

The documentation for union_relations should call out that the generated SQL uses union all. By the macro name, folks might assume it uses union (which is semantically different than union all).

Describe alternatives you've considered

N/A

Additional context

N/A

Who will this benefit?

Folks trying to understand the capabilities and limitations of this package's macros

Are you interested in contributing this feature?

Sure!