update docs, add load_all(), tests for load_all, make through field optional

docs/api/models/helpers/related-names-validation.md

@@ -0,0 +1,25 @@
+<a name="models.helpers.related_names_validation"></a>
+# models.helpers.related\_names\_validation
+
+<a name="models.helpers.related_names_validation.validate_related_names_in_relations"></a>
+#### validate\_related\_names\_in\_relations
+
+```python
+validate_related_names_in_relations(model_fields: Dict, new_model: Type["Model"]) -> None
+```
+
+Performs a validation of relation_names in relation fields.
+If multiple fields are leading to the same related model
+only one can have empty related_name param
+(populated by default as model.name.lower()+'s').
+Also related_names have to be unique for given related model.
+
+**Raises**:
+
+- `ModelDefinitionError`: if validation of related_names fail
+
+**Arguments**:
+
+- `model_fields (Dict[str, ormar.Field])`: dictionary of declared ormar model fields
+- `new_model (Model class)`: 
+