Clarifying BigQuery unit test (#5367)

## What are you changing in this pull request and why?

Closes #5331
Closes #5307

## Checklist
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---------

Co-authored-by: Leona B. Campbell <[email protected]>

website/docs/docs/build/unit-tests.md

@@ -22,8 +22,9 @@ Now, we are introducing a new type of test to dbt - unit tests. In software prog
 ## Before you begin
 
 - We currently only support unit testing SQL models.
-- We currently don't support unit testing models that use recursive SQL. 
 - We currently only support adding unit tests to models in your _current_ project.
+- We currently *don't* support unit testing models that use recursive SQL.
+- You must specify all fields in a BigQuery STRUCT in a unit test. You cannot use only a subset of fields in a STRUCT.
 - If your model has multiple versions, by default the unit test will run on *all* versions of your model. Read [unit testing versioned models](#unit-testing-versioned-models) for more information.
 - Unit tests must be defined in a YML file in your `models/` directory.
 

website/docs/reference/resource-properties/data-types.md

@@ -64,6 +64,9 @@ unit_tests:
            struct_field: 'struct("Isha" as name, 22 as age)'
            struct_of_struct_field: 'struct(struct(1 as id, "blue" as color) as my_struct)'
            struct_array_field: ['struct(st_geogpoint(75, 45) as my_point)', 'struct(st_geogpoint(75, 35) as my_point)']
+           # Make sure to include all the fields in a BigQuery `struct` within the unit test.
+           # It's not currently possible to use only a subset of columns in a 'struct'
+
 
 ```