update(docs): improve Jest cookbook and troubleshooting for coverage (#228)

* Clarify Generate code coverage reports for jest
* Add Missing Jest coverage data to troubleshooting

Author: ArturT

docusaurus/docs/jest/cookbook.md

@@ -44,13 +44,49 @@ npx knapsack-pro-jest --config=jest.config.e2e.js
 
 ## Generate code coverage reports
 
+Each CI node executes multiple batches of tests in Knapsack Pro [Queue Mode](../overview/index.mdx#queue-mode-dynamic-split), and each batch starts a new Jest process. By default, a coverage report generated by each batch overwrites the previous one, which can lead to incomplete coverage data.
+
+To avoid this, you can configure Knapsack Pro to generate a separate coverage report for each batch by setting the following environment variable:
+
 ```bash
 export KNAPSACK_PRO_COVERAGE_DIRECTORY=coverage
 
 npx knapsack-pro-jest --coverage
 ```
 
-Knapsack Pro generates one coverage report for each batch of tests executed on the CI node. To merge the reports you may consider [this script](https://github.com/jestjs/jest/issues/2418#issuecomment-478932514).
+This ensures each batch creates a unique subfolder for its coverage report, preventing overwrites:
+
+```
+coverage
+    ├── 04c9ea40-4477-11ea-8397-c539e4406df8
+    │   ├── clover.xml
+    │   ├── coverage-final.json
+    │   ├── lcov-report
+    │   │   ├── a.js.html
+    │   │   ├── base.css
+    │   │   ├── block-navigation.js
+    │   │   ├── index.html
+    │   │   ├── prettify.css
+    │   │   ├── prettify.js
+    │   │   ├── sort-arrow-sprite.png
+    │   │   └── sorter.js
+    │   └── lcov.info
+    └── 0c584590-4477-11ea-8397-c539e4406df8
+        ├── clover.xml
+        ├── coverage-final.json
+        ├── lcov-report
+        │   ├── b.js.html
+        │   ├── base.css
+        │   ├── block-navigation.js
+        │   ├── index.html
+        │   ├── prettify.css
+        │   ├── prettify.js
+        │   ├── sort-arrow-sprite.png
+        │   └── sorter.js
+        └── lcov.info
+```
+
+You can merge the reports with [this script](https://github.com/jestjs/jest/issues/2418#issuecomment-478932514).
 
 ## Generate XML reports
 

docusaurus/docs/jest/troubleshooting.md

@@ -48,3 +48,7 @@ npx knapsack-pro-jest --runInBand
 ## No tests are executed (or `test_files: [ 'parameter is required' ]`)
 
 Make sure [`KNAPSACK_PRO_TEST_FILE_PATTERN`](reference.md#knapsack_pro_test_file_pattern) is correct.
+
+## Missing Jest coverage data
+
+See [how to configure Jest code coverage](cookbook.md#generate-code-coverage-reports).