Add docs for PLC rules (#4224)

crates/ruff/src/rules/pylint/rules/compare_to_empty_string.rs

@@ -51,6 +51,30 @@ impl std::fmt::Display for EmptyStringCmpop {
     }
 }
 
+/// ## What it does
+/// Checks for comparisons to empty strings.
+///
+/// ## Why is this bad?
+/// An empty string is falsy, so it is unnecessary to compare it to `""`. If
+/// the value can be something else Python considers falsy, such as `None` or
+/// `0` or another empty container, then the code is not equivalent.
+///
+/// ## Example
+/// ```python
+/// def foo(x):
+///     if x == "":
+///         print("x is empty")
+/// ```
+///
+/// Use instead:
+/// ```python
+/// def foo(x):
+///     if not x:
+///         print("x is empty")
+/// ```
+///
+/// ## References
+/// - [Python documentation](https://docs.python.org/3/library/stdtypes.html#truth-value-testing)
 #[violation]
 pub struct CompareToEmptyString {
     pub existing: String,

crates/ruff/src/rules/pylint/rules/unnecessary_direct_lambda_call.rs

@@ -5,6 +5,25 @@ use ruff_macros::{derive_message_formats, violation};
 
 use crate::checkers::ast::Checker;
 
+/// ## What it does
+/// Checks for unnecessary direct calls to lambda expressions.
+///
+/// ## Why is this bad?
+/// Calling a lambda expression directly is unnecessary. The expression can be
+/// executed inline instead to improve readability.
+///
+/// ## Example
+/// ```python
+/// area = (lambda r: 3.14 * r ** 2)(radius)
+/// ```
+///
+/// Use instead:
+/// ```python
+/// area = 3.14 * radius ** 2
+/// ```
+///
+/// ## References
+/// - [Python documentation](https://docs.python.org/3/reference/expressions.html#lambda)
 #[violation]
 pub struct UnnecessaryDirectLambdaCall;
 

crates/ruff/src/rules/pylint/rules/useless_import_alias.rs

@@ -6,6 +6,21 @@ use ruff_macros::{derive_message_formats, violation};
 use crate::checkers::ast::Checker;
 use crate::registry::AsRule;
 
+/// ## What it does
+/// Checks for import aliases that do not rename the original package.
+///
+/// ## Why is this bad?
+/// The import alias is redundant and should be removed to avoid confusion.
+///
+/// ## Example
+/// ```python
+/// import numpy as numpy
+/// ```
+///
+/// Use instead:
+/// ```python
+/// import numpy as np
+/// ```
 #[violation]
 pub struct UselessImportAlias;