Clarify the factor-conditional dependency_groups comment in tox.ini

Author: tpvasconcelos

tox.ini

@@ -28,9 +28,16 @@ passenv =
     GITHUB_*
     PYTEST_*
 parallel_show_output = true
-# Only install the `tests` dependency group for the test and coverage
-# envs (other envs inheriting from this section, such as `build-dists`
-# and `pre-commit-*`, define their own dependencies)
+# Install the `tests` dependency group (from pyproject.toml), but only in
+# the envs that run tests or coverage. The `pytest,tests,coverage:` prefix
+# is tox's factor-conditional syntax (also used in `commands` below): the
+# value after the colon only applies to envs matching one of these factors,
+# i.e. `pytest`, `tests-{unit,e2e,cicd_utils}`, and `coverage-combine`.
+# Without this guard, envs that inherit from this section but don't override
+# this key (e.g. `build-dists` and `pre-commit-*`) would also install the
+# test dependencies. NOTE: the `tests` factor (left of the colon) and the
+# `tests` dependency group (right of the colon) are different things that
+# just happen to share a name.
 dependency_groups =
     pytest,tests,coverage: tests
 allowlist_externals = rm, mv