Multiple foreign key constraints can exist on the same column

[ericharmeling]

Fixes #6607.

• Added notes and example to Foreign Key docs page.
• Removed single-column requirement from Foreign Key docs page.
• Removed note from Performance docs.

cockroachdb/cockroach#43417 affects more than just docs about foreign keys... In specific, we need to update the MovR schema to take advantage of this lifted restriction. That schema needs to be updated for a number of other things as well, so I consider updating the schema out of the scope of this docs issue.

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

Online preview: http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/9d56065e99ec02d3a5ea6a151dff4e9d1cdf00a0/

Edited pages:

• v20.1/foreign-key.md

[jseldess]

Thanks, @ericharmeling!

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @jseldess)