Defining multiple primary_key constraints at the column level is no…

…t supported (#5210)

## What are you changing in this pull request and why?
Revises the verbiage based on a recent conversation with the core team,
where multiple column-level primary key constraints actually fails
silently in 1.7, but will generate a warning in 1.8 as unsupported.

We are not clear about this today in docs, leading to potential
confusion and technical debt for users on older versions.

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website/docs/reference/resource-properties/constraints.md

@@ -15,6 +15,8 @@ Constraints require the declaration and enforcement of a model [contract](/refer
 
 Constraints may be defined for a single column, or at the model level for one or more columns. As a general rule, we recommend defining single-column constraints directly on those columns.
 
+If you are defining multiple `primary_key` constraints for a single model, those _must_ be defined at the model level. Defining multiple `primary_key` constraints at the column level is not supported. 
+
 The structure of a constraint is:
 - `type` (required): one of `not_null`, `unique`, `primary_key`, `foreign_key`, `check`, `custom`
 - `expression`: Free text input to qualify the constraint. Required for certain constraint types, and optional for others.