Added missing docs for clear_cache! on adapters

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -332,6 +332,7 @@ module ActiveRecord
         rows
       end
 
+      # Clear prepared statement cache.
       def clear_cache!
         @statements.values.each do |cache|
           cache[:stmt].close

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -237,6 +237,7 @@ module ActiveRecord
         @local_tz = execute('SHOW TIME ZONE', 'SCHEMA').first["TimeZone"]
       end
 
+      # Clear prepared statement cache.
       def clear_cache!
         @statements.each_value do |value|
           @connection.query "DEALLOCATE #{value}"

activerecord/lib/active_record/connection_adapters/sqlite_adapter.rb

@@ -96,6 +96,7 @@ module ActiveRecord
         @connection.close rescue nil
       end
 
+      # Clear prepared statement cache.
       def clear_cache!
         @statements.clear
       end