pacman: improve docs, make sure that packages is always returned, deprecate update_cache behavior (#4330) (#4342)

* Improve docs, make sure that packages is always returned, deprecate update_cache behavior.

* Add cache_updated return value.

* Revert "Add cache_updated return value."

This reverts commit 367297bb5c.

* Update tests/unit/plugins/modules/packaging/os/test_pacman.py

Co-authored-by: Jean Raby <jean@raby.sh>

Co-authored-by: Jean Raby <jean@raby.sh>
(cherry picked from commit 10ca62905f)

Co-authored-by: Felix Fontein <felix@fontein.de>

tests/unit/plugins/modules/packaging/os/test_pacman.py

@@ -325,6 +325,7 @@ class TestPacman:
             P.run()
         self.mock_run_command.call_count == 0
         out = e.value.args[0]
+        assert "packages" not in out
         assert out["changed"]
 
     @pytest.mark.parametrize(
@@ -582,8 +583,8 @@ class TestPacman:
         with pytest.raises(AnsibleExitJson) as e:
             P.run()
         out = e.value.args[0]
-        assert "packages" not in out
         assert not out["changed"]
+        assert "packages" in out
         assert "diff" not in out
         self.mock_run_command.call_count == 0
 
@@ -1010,6 +1011,7 @@ class TestPacman:
         if raises == AnsibleExitJson:
             assert out["packages"] == expected_packages
             assert out["changed"]
+            assert "packages" in out
             assert "diff" in out
         else:
             assert out["stdout"] == "stdout"