Improve the example of ActiveSupport::Cache::Store#fetch

The original example has race condition issue that the output of the example isn't conistent, see rails#43588.

The change improves the example by removing unnecessary thread and add detailed comments.

In the new example, there will be race condition only if `ActiveSupport::Cache::MemoryStore` took longer than 0.1 second to extend expiry but that's unlikely to happen.

Close rails#43588

Author: tonytonyjan

activesupport/lib/active_support/cache.rb

@@ -405,31 +405,47 @@ def mute
       #   has elapsed.
       #
       #     # Set all values to expire after one minute.
-      #     cache = ActiveSupport::Cache::MemoryStore.new(expires_in: 1.minute)
+      #     cache = ActiveSupport::Cache::MemoryStore.new(expires_in: 1)
       #
-      #     cache.write('foo', 'original value')
+      #     cache.write("foo", "original value")
       #     val_1 = nil
       #     val_2 = nil
-      #     sleep 60
+      #     p cache.read("foo") # => "original value"
       #
-      #     Thread.new do
-      #       val_1 = cache.fetch('foo', race_condition_ttl: 10.seconds) do
+      #     sleep 1 # wait until the cache expires
+      #
+      #     t1 = Thread.new do
+      #       # fetch does the following:
+      #       # 1. gets an recent expired entry
+      #       # 2. extends the expiry by 2 seconds (race_condition_ttl)
+      #       # 3. regenerates the new value
+      #       val_1 = cache.fetch("foo", race_condition_ttl: 2) do
       #         sleep 1
-      #         'new value 1'
+      #         "new value 1"
       #       end
       #     end
       #
-      #     Thread.new do
-      #       val_2 = cache.fetch('foo', race_condition_ttl: 10.seconds) do
-      #         'new value 2'
-      #       end
+      #     # Wait until t1 extends the expiry of the entry
+      #     # but before generating the new value
+      #     sleep 0.1
+      #
+      #     val_2 = cache.fetch("foo", race_condition_ttl: 2) do
+      #       # This block won't be executed because t1 extended the expiry
+      #       "new value 2"
       #     end
       #
-      #     cache.fetch('foo') # => "original value"
-      #     sleep 10 # First thread extended the life of cache by another 10 seconds
-      #     cache.fetch('foo') # => "new value 1"
-      #     val_1 # => "new value 1"
-      #     val_2 # => "original value"
+      #     t1.join
+      #
+      #     p val_1 # => "new value 1"
+      #     p val_2 # => "oritinal value"
+      #     p cache.fetch("foo") # => "new value 1"
+      #
+      #     # The entry requires 3 seconds to expire (expires_in + race_condition_ttl)
+      #     # We have waited 2 seconds already (sleep(1) + t1.join) thus we need to wait 1
+      #     # more second to see the entry expire.
+      #     sleep 1
+      #
+      #     p cache.fetch("foo") # => nil
       #
       # ==== Dynamic Options
       #