Merge pull request #20099 from yui-knk/fix/comment_association_cache

[ci skip] Fix comment of `ActiveRecord::Associations#association_inst…

activerecord/lib/active_record/associations.rb

@@ -185,7 +185,7 @@ def init_internals # :nodoc:
         super
       end
 
-      # Returns the specified association instance if it responds to :loaded?, nil otherwise.
+      # Returns the specified association instance if it exists, nil otherwise.
       def association_instance_get(name)
         @association_cache[name]
       end