Add allow(missing_docs) attribute

The code generated for mapping_definitions was causing linting issues
when using the ElasticType derive attribute.

Author: mwilliammyers

src/types_derive_internals/src/elastic_type/mod.rs

@@ -64,6 +64,7 @@ pub fn expand_derive(
     let mapping_impl_block = &mapping.impl_block;
 
     Ok(vec![quote!(
+        #[allow(missing_docs)]
         #mapping_definition
 
         #[allow(non_upper_case_globals, dead_code, unused_variables)]