Add docs for f-string-missing-placeholders and unused-variable (#2790)

crates/ruff/src/rules/pyflakes/rules/f_string_missing_placeholders.rs

@@ -8,6 +8,26 @@ use crate::registry::Diagnostic;
 use crate::violation::AlwaysAutofixableViolation;
 
 define_violation!(
+    /// ## What it does
+    /// Checks for f-strings that do not contain any placeholder expressions.
+    ///
+    /// ## Why is this bad?
+    /// F-strings are a convenient way to format strings, but they are not
+    /// necessary if there are no placeholder expressions to format. In this case,
+    /// a regular string should be used instead.
+    ///
+    /// ## Example
+    /// ```python
+    /// f"Hello, world!"
+    /// ```
+    ///
+    /// Use instead:
+    /// ```python
+    /// "Hello, world!"
+    /// ```
+    ///
+    /// ## References
+    /// * [PEP 498](https://www.python.org/dev/peps/pep-0498/)
     pub struct FStringMissingPlaceholders;
 );
 impl AlwaysAutofixableViolation for FStringMissingPlaceholders {

crates/ruff/src/rules/pyflakes/rules/unused_variable.rs

@@ -16,6 +16,31 @@ use crate::source_code::Locator;
 use crate::violation::AlwaysAutofixableViolation;
 
 define_violation!(
+    /// ## What it does
+    /// Checks for the presence of unused variables in function scopes.
+    ///
+    /// ## Why is this bad?
+    /// A variable that is defined but not used is likely a mistake, and should be
+    /// removed to avoid confusion.
+    ///
+    /// If a variable is intentionally defined-but-not-used, it should be prefixed
+    /// with an underscore, or some other value that adheres to the
+    /// [`dummy-variable-rgx`](https://github.com/charliermarsh/ruff#dummy-variable-rgx) pattern.
+    ///
+    /// ## Example
+    /// ```python
+    /// def foo():
+    ///     x = 1
+    ///     y = 2
+    ///     return x
+    /// ```
+    ///
+    /// Use instead:
+    /// ```python
+    /// def foo():
+    ///     x = 1
+    ///     return x
+    /// ```
     pub struct UnusedVariable {
         pub name: String,
     }