docs(dev/guide): fix inaccurate single-letter rule references

Verified the guidance against `perfectionist` 0.0.0-rc.17 and corrected
three claims that did not match the rules:

- Remove the fabricated `short_impl_max_lines` knob from the generic
  naming section. The `single_letter_generic` rule has no configuration;
  canonical cases are silenced site-by-site with `#[allow]` or
  `#[expect]`.
- Rename the `allowed_idents` knob to `extra_allowed_idents`, which is
  the actual field name for the variable and closure rules.
- Clarify that `i`/`j`/`k` are exempt as function and closure parameters
  but not as `let` bindings, where only `n` is allowed.

https://claude.ai/code/session_01CMrfXtuzxpSbchSJNuwboK

Author: claude

CONTRIBUTING.md

@@ -89,11 +89,11 @@ Use **descriptive names** for type parameters, not single letters:
 
 - `Size`, `Name`, `SizeGetter`, `HardlinksRecorder`, `Report`
 
-Single-letter generics are acceptable only in very short, self-contained trait impls. Enforced by `perfectionist::single_letter_generic`; the threshold for "very short" is the rule's `short_impl_max_lines` knob in `dylint.toml`.
+Single-letter type parameters are flagged by `perfectionist::single_letter_generic`, which has no configuration. Genuinely canonical cases, such as `impl<T> From<T> for Wrapper<T>` where the trait already fixes the role of `T`, may be silenced site-by-site with `#[allow]` or `#[expect]`.
 
 ### Variable and Closure Parameter Naming
 
-Use **descriptive names** for variables and closure parameters by default. Single-letter names are permitted only in the specific cases listed below. Enforced by `perfectionist::single_letter_let_binding`, `perfectionist::single_letter_function_param`, and `perfectionist::single_letter_closure_param`; the per-rule `allowed_idents` and `extra_trivial_callback_methods` knobs in `dylint.toml` reflect the exceptions documented here.
+Use **descriptive names** for variables and closure parameters by default. Single-letter names are permitted only in the specific cases listed below. Enforced by `perfectionist::single_letter_let_binding`, `perfectionist::single_letter_function_param`, and `perfectionist::single_letter_closure_param`. The exact exemptions differ by binding kind, as the cases below describe. The `extra_allowed_idents` and `extra_trivial_callback_methods` knobs in `dylint.toml` extend the built-in exempt sets, though the project currently relies on the defaults aside from the `sort_reflection_by` callback.
 
 #### When single-letter names are allowed
 
@@ -110,16 +110,16 @@ Use **descriptive names** for variables and closure parameters by default. Singl
   fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { todo!() }
   ```
 
-- **Index variables (`i`, `j`, `k`):** These may only be used in two contexts: short closures, and index-based loops or iterations. The latter is rare in Rust. In all other cases, use `index`, `idx`, or `*_index`.
+- **Index variables (`i`, `j`, `k`):** These are exempt as function and closure parameters, and they read naturally in index-based loops or iterations, which are rare in Rust. They are not exempt as `let` bindings, where only `n` is allowed, so a `let` that holds an index must use `index`, `idx`, or `*_index` instead.
 
   ```rust
-  // OK: short closure
+  // OK: closure parameter
   left_indices.zip(right_indices).map(|(i, j)| matrix[i][j])
 
   // OK: index-based loop
   for i in 0..len { /* ... */ }
 
-  // Bad: use a descriptive name instead
+  // Bad: a `let` binding allows only `n`, never `i`
   let i = items.iter().position(|item| item.is_active()).unwrap();
   ```