Build/Test Tools: Add inline docs to explain the PHPUnit strategy.

In [59587], the PHPUnit workflow strategy matrix was expanded using the criteria of “all LTS versions supported by WordPress with `>1%` usage according to WordPress.org/stats,” but this was only mentioned in the commit message.

This adds some inline documentation to the explain that methodology for future reference.

See #63170.

git-svn-id: https://develop.svn.wordpress.org/trunk@60318 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

.github/workflows/phpunit-tests.yml

@@ -53,6 +53,10 @@ jobs:
   #
   # Creates a PHPUnit test job for each PHP/MySQL combination.
   #
+  # Though MySQL 5.5 and 5.6 are still supported by WordPress, they are not currently tested here because the Docker
+  # images do not work. Testing against MariaDB 5.5 provides a reasonable level of MySQL 5.5 testing (see MariaDB matrix
+  # below for more details).
+  #
   test-with-mysql:
     name: PHP ${{ matrix.php }}
     uses: ./.github/workflows/reusable-phpunit-tests-v3.yml
@@ -124,6 +128,10 @@ jobs:
   #
   # Creates a PHPUnit test job for each PHP/MariaDB combination.
   #
+  # All LTS versions of MariaDB supported by WordPress with greater than 1% usage according to w.org/stats should be
+  # tested. The exception to this rule is version 5.5. That release was intended as a drop-in replacement for MySQL.
+  # Because the MySQL 5.5 Docker containers do not work, this ensures some level of MySQL 5.5 testing.
+  #
   test-with-mariadb:
     name: PHP ${{ matrix.php }}
     uses: ./.github/workflows/reusable-phpunit-tests-v3.yml
@@ -168,11 +176,11 @@ jobs:
   #
   # Creates PHPUnit test jobs to test MariaDB and MySQL innovation releases.
   #
-  # Though innovation releases are deemed "production grade" and never receive LTS status, they include new features
-  # and updates that will be included in the next LTS version.
+  # Though innovation releases are deemed "production grade", they never receive LTS status. However, they include new
+  # features and updates that will be included in the next LTS version.
   #
-  # Because upstream support for innovation releases gets dropped when a new one is released, only the most recent
-  # innovation version is tested.
+  # Because upstream support for innovation releases is dropped when a new one is released (including security updates),
+  # only the most recent innovation version is tested.
   #
   test-innovation-releases:
     name: PHP ${{ matrix.php }}
@@ -217,6 +225,8 @@ jobs:
   #
   # Runs specific individual test groups.
   #
+  # These tests are run against the most recent LTS version of MySQL.
+  #
   specific-test-groups:
     name: ${{ matrix.phpunit-test-groups }}
     uses: ./.github/workflows/reusable-phpunit-tests-v3.yml