Add notes on when to add ignores to warnings (#8987)

* Add notes on when to add ignores to warnings

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

pyproject.toml

@@ -288,18 +288,22 @@ addopts = ["--strict-config", "--strict-markers"]
 # - Converts any warning from xarray into an error
 # - Allows some warnings ("default") which the test suite currently raises,
 #   since it wasn't practical to fix them all before merging this config. The
-#   arnings are still listed in CI (since it uses `default`, not `ignore`).
+#   warnings are reported in CI (since it uses `default`, not `ignore`).
 #
-# We can remove these rules allowing warnings; a valued contribution is removing
-# a line, seeing what breaks, and then fixing the library code or tests so that
-# it doesn't raise warnings.
+# Over time, we can remove these rules allowing warnings. A valued contribution
+# is removing a line, seeing what breaks, and then fixing the library code or
+# tests so that it doesn't raise warnings.
 #
-# While we only raise an error on warnings from within xarray, if dependency
-# raises a warning with a stacklevel such that it's interpreted to be raised
-# from xarray, please feel free to add a rule switching it to `default` here.
-#
-# If these settings get in the way of making progress, it's also acceptable to
-# temporarily add additional ignores.
+# There are some instance where we'll want to add to these rules:
+# - While we only raise errors on warnings from within xarray, a dependency can
+#   raise a warning with a stacklevel such that it's interpreted to be raised
+#   from xarray and this will mistakenly convert it to an error. If that
+#   happens, please feel free to add a rule switching it to `default` here, and
+#   disabling the error.
+# - If these settings get in the way of making progress, it's also acceptable to
+#   temporarily add additional `default` rules.
+# - But we should only add `ignore` rules if we're confident that we'll never
+#   need to address a warning.
 
 filterwarnings = [
   "error:::xarray.*",