docs: note that numeric cases filtering requires Sphinx-Needs >= 6 (#139)

## Summary

Follow-up to #136. That PR switched the filter examples to direct
numeric
comparisons (`cases > 4`, `cases >= 5`), which is correct for
**Sphinx-Needs >= 6**:
there the count fields are registered with an integer schema
(`sphinxcontrib/test_reports/test_reports.py:245-250`) and the
directives pass
real `int` values.

On Sphinx-Needs < 6 (the `else` branch at `test_reports.py:260-266`)
those
fields have no integer schema and are stored as strings, so `cases > 4`
raises a
`TypeError` during filtering — which is why the original
`cases.isdigit() and int(cases) > 4` form was used. The two forms are
version-specific inverses, so this is a documentation note rather than a
code
change.

This adds a `.. note::` to `docs/filter.rst` stating that numeric
filtering
requires Sphinx-Needs >= 6 and pointing users on older versions to the
`isdigit()` / `int()` form.

## Alternative

The runtime dependency floor is still `sphinx-needs>=1.0.1`
(`pyproject.toml`),
while the test extra already pins `>=6`. If support for Sphinx-Needs < 6
is no
longer intended, an alternative to this note would be to bump the
runtime floor
to `>=6` and drop the legacy `else` branch in `sphinx_needs_update()`.
Happy to
do that instead if preferred.

## Validation

- Documentation-only change; no code paths touched.

Author: ubmarco

docs/filter.rst

@@ -92,6 +92,14 @@ Usage::
 ``filter`` supports complex-filter operations by using a Python-statement.
 In this case, we check that the ``cases`` value is greater than 4.
 
+.. note::
+
+   Numeric comparisons such as ``cases > 4`` require Sphinx-Needs 6.0 or newer,
+   where the count fields (``suites``, ``cases``, ``passed``, ``skipped``,
+   ``failed`` and ``errors``) are stored as integers. On older Sphinx-Needs
+   versions these values are strings, so use
+   ``cases.isdigit() and int(cases) > 4`` instead.
+
 Take a look into the
 `Filter string section <https://sphinx-needs.readthedocs.io/en/latest/filter.html#filter-string>`_
 of the Sphinx-Needs documentations for more details and ideas how to use it.