doc: clarify that union_relations() uses union all

[owenprough-sift]

resolves #759

This is a:

• documentation update
• bug fix with no breaking changes
• new functionality
• a breaking change

All pull requests from community contributors should target the main branch (default).

Description & motivation

SQL supports two primary ways to combine query results: union (de-duping row-wise) and union all (no de-duping). The documentation for the union_relations() macro should be clear about which is used here.

Checklist

• This code is associated with an Issue which has been triaged and accepted for development.
• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I followed guidelines to ensure that my changes will work on "non-core" adapters by:
  • dispatching any new macro(s) so non-core adapters can also use them (e.g. the star() source)
  • using the limit_zero() macro in place of the literal string: limit 0
  • using dbt.type_* macros instead of explicit datatypes (e.g. dbt.type_timestamp() instead of TIMESTAMP
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md

[owenprough-sift]

Do trivial documentation updates need entries in CHANGELOG.md?

[joellabes]

Love it! Thank you @owenprough-sift 🙏

In general we don't changelog small documentation changes, but it's not a strictly enforced thing either way!