Fix #2578: Allow instances of generic data tests to be documented

[aranke]

Resolves #2578

Problem

Not being able to document instances of generic data tests has been a longstanding paper cut.

Solution

Allow documenting instances of generic data tests, e.g.

- name: color
  tests:
    - accepted_values:
        values: ['blue', 'green', 'red']
        description: "{{ doc('color_accepted_values') }}"

Tests implemented:

• Setting description as a string
• No description set, defaults to ""
• Description can be set via doc block

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.13%. Comparing base (40c350f) to head (22c8d63).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10850      +/-   ##
==========================================
- Coverage   89.18%   89.13%   -0.06%     
==========================================
  Files         183      183              
  Lines       23429    23433       +4     
==========================================
- Hits        20895    20886       -9     
- Misses       2534     2547      +13     

Flag	Coverage Δ
integration	86.36% <100.00%> (-0.13%)	⬇️
unit	62.12% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	62.12% <100.00%> (+<0.01%)	⬆️
Integration Tests	86.36% <100.00%> (-0.13%)	⬇️

[gshank]

Nice!