Update CustomAttributeMetadata endpoint topic to include attribute_options field

[keharper]

Type of Doc:

• New topic
• Topic Update, Devdoc link: https://devdocs.magento.com/guides/v2.3/graphql/reference/custom-attribute-metadata.html
• Other

Magento 2 GitHub Issue: magento/graphql-ce#87

Magento 2 GitHub PR: magento/graphql-ce#108

Magento 2 version:

• 2.1.X
• 2.2.X
• 2.3.X
• back port
• forward port

Notifications of Changes:

• Changes to Admin interface?
• New software requirements?
• New REST endpoints or updated GraphQL query syntax?
• New command line option?

Content:

Document the attribute_options field and update the example.

[keharper]

Work completed by #2870