Add documentation for GraphQL Union field bug

[Orrison]

Ticket(s) or GitHub Issue

• https://canyongbs.atlassian.net/browse/ADVAPP-115

Technical Description

There is no fix for this issue. It comes from our usage of Union types that share the same field names but no the same type definition for that field. There is currently no way around this, and it is much discussed in the GraphQL community. graphql/graphql-js#53

Technically, the specification is not hard on this limitation being required. So, I have opened an issue in the underlying PHP GraphQL package to see if it can be fixed. webonyx/graphql-php#1500

But I have added some documentation for now for the workaround.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Content or styling update (Changes which don't affect functionality)
• DevOps
• Documentation

Screenshots (if appropriate)

Any deployment steps required?

A deployment step could be a command that needs to be executed or an ENV key that needs to be added, for example.

• Yes, please specify
• No

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