Document child entity filtering (#159)

* Document child entity filtering

* Use "nested" instead of "child"

Author: kamilkisiela

pages/en/querying/graphql-api.mdx

@@ -150,11 +150,30 @@ This can be useful if you are looking to fetch only entities which have changed,
 }
 ```
 
+#### Example for nested entity filtering
+
+Filtering on the basis of nested entities is possible in the fields with the `_` suffix.
+
+This can be useful if you are looking to fetch only entities whose child-level entities meet the provided conditions.
+
+```graphql
+{
+  challenges(where: { application_: { id: 1 } }) {
+    challenger
+    outcome
+    application {
+      id
+    }
+  }
+}
+```
+
 #### All Filters
 
 Full list of parameter suffixes:
 
 ```
+_
 _not
 _gt
 _lt
@@ -176,7 +195,7 @@ _not_ends_with
 _not_ends_with_nocase
 ```
 
-> Please note that some suffixes are only supported for specific types. For example, `Boolean` only supports `_not`, `_in`, and `_not_in`.
+> Please note that some suffixes are only supported for specific types. For example, `Boolean` only supports `_not`, `_in`, and `_not_in`, but `_` is available only for object and interface types.
 
 In addition, the following global filters are available as part of `where` argument: