[Fix #11803] Update the doc for Style/RedundantFetchBlock

Fixes #11803.

This PR adds a note to the doc for `Style/RedundantFetchBlock` about the difference
in behavior between enabled and disabled frozen string magic literal comment.

lib/rubocop/cop/style/redundant_fetch_block.rb

@@ -3,11 +3,13 @@
 module RuboCop
   module Cop
     module Style
-      # Identifies places where `fetch(key) { value }`
-      # can be replaced by `fetch(key, value)`.
+      # Identifies places where `fetch(key) { value }` can be replaced by `fetch(key, value)`.
       #
-      # In such cases `fetch(key, value)` method is faster
-      # than `fetch(key) { value }`.
+      # In such cases `fetch(key, value)` method is faster than `fetch(key) { value }`.
+      #
+      # NOTE: The block string `'value'` in `hash.fetch(:key) { 'value' }` is detected
+      # when frozen string literal magic comment is enabled (i.e. `# frozen_string_literal: true`),
+      # but not when disabled.
       #
       # @safety
       #   This cop is unsafe because it cannot be guaranteed that the receiver