Update user_guide_src/source/database/query_builder.rst
Update user_guide_src/source/database/query_builder.rst

Clarification for union() method.

Co-authored-by: Michal Sniatala <a2df1e659c9fd2578de0a26565357cb273292eeb@sniatala.pl>
kenjis/CodeIgniter4,kenjis/CodeIgniter4,kenjis/CodeIgniter4,kenjis/CodeIgniter4,kenjis/CodeIgniter4