Document task environment variable parameters

Some tasks accept input via environment variables. It is important to document these parameter variables.

Previously, the parameter environment variables of these tasks were undocumented.

Typically, the documentation is done in the task description, to make the information easily accessible to contributors
via the `task list` output. However, this approach was intentionally eschewed in the case of the `yaml:lint` task, where
the documentation was instead placed in a comment. The reason is that this task's parameter variable is only useful when
the task is executed by a GitHub Actions workflow, as is done already in the "Check YAML" workflow. So the contributor
running the task from the command line has no need for this information and thus including it in the description would
only clutter up the `task list` output with content useless to the reader of that output.

Author: per1234

Taskfile.yml

@@ -342,7 +342,11 @@ tasks:
 
   # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/test-go-task/Taskfile.yml
   go:test:
-    desc: Run unit tests
+    desc: |
+      Run unit tests.
+      Environment variable parameters:
+      - GO_MODULE_PATH: Path of the Go module root (default: {{.DEFAULT_GO_MODULE_PATH}}).
+      - GO_PACKAGES: List of Go packages to test (default: all packages of the module).
     dir: "{{default .DEFAULT_GO_MODULE_PATH .GO_MODULE_PATH}}"
     cmds:
       - |