Add PT013 and PT015 docs (#6303)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

#2646

## Test Plan

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

@@ -112,6 +112,29 @@ impl Violation for PytestAssertInExcept {
     }
 }
 
+/// ## What it does
+/// Checks for `assert` statements whose test expression is a falsy value.
+///
+/// ## Why is this bad?
+/// `pytest.fail` conveys the intent more clearly than `assert falsy_value`.
+///
+/// ## Example
+/// ```python
+/// def test_foo():
+///     if some_condition:
+///         assert False, "some_condition was True"
+/// ```
+///
+/// Use instead:
+/// ```python
+/// def test_foo():
+///     if some_condition:
+///         pytest.fail("some_condition was True")
+///     ...
+/// ```
+///
+/// References
+/// - [`pytest` documentation: `pytest.fail`](https://docs.pytest.org/en/latest/reference/reference.html#pytest-fail)
 #[violation]
 pub struct PytestAssertAlwaysFalse;
 

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

@@ -3,6 +3,23 @@ use ruff_python_ast::{Ranged, Stmt};
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
 
+/// ## What it does
+/// Checks for incorrect import of pytest.
+///
+/// ## Why is this bad?
+/// `pytest` should be imported as `import pytest` and its members should be accessed in the form of
+/// `pytest.xxx.yyy` for consistency and to make it easier for linting tools to analyze the code.
+///
+/// ## Example
+/// ```python
+/// import pytest as pt
+/// from pytest import fixture
+/// ```
+///
+/// Use instead:
+/// ```python
+/// import pytest
+/// ```
 #[violation]
 pub struct PytestIncorrectPytestImport;