Add `PT020` doc (#6011)

crates/ruff/src/rules/flake8_pytest_style/rules/fixture.rs

@@ -114,6 +114,38 @@ impl Violation for PytestFixtureParamWithoutValue {
     }
 }
 
+/// ## What it does
+/// Checks for `pytest.yield_fixture` usage.
+///
+/// ## Why is this bad?
+/// `pytest.yield_fixture` is deprecated. `pytest.fixture` should be used instead.
+///
+/// ## Example
+/// ```python
+/// import pytest
+///
+///
+/// @pytest.yield_fixture()
+/// def my_fixture():
+///     obj = SomeClass()
+///     yield obj
+///     obj.cleanup()
+/// ```
+///
+/// Use instead:
+/// ```python
+/// import pytest
+///
+///
+/// @pytest.fixture()
+/// def my_fixture():
+///     obj = SomeClass()
+///     yield obj
+///     obj.cleanup()
+/// ```
+///
+/// ## References
+/// - [`yield_fixture` functions](https://docs.pytest.org/en/latest/yieldfixture.html)
 #[violation]
 pub struct PytestDeprecatedYieldFixture;