docs(graphql): added every relationship available (#113)

(cherry picked from commit a7b9f326e4)

docs/guide/graphql/filtering/index.md

@@ -65,8 +65,10 @@ query {
         data {
             name
             animethemes(type: OP) {
-                type
-                sequence
+                data {
+                    type
+                    sequence
+                }
             }
         }
     }

docs/guide/graphql/relationships/index.md

@@ -8,9 +8,45 @@ title: Relationships
 
 The AnimeThemes API implements relationships for most types, previously known as allowed included in the REST API.
 
-// TODO: Waiting API refactor: pagination everywhere.
+The many-to-many, one-to-many and polymorphic one-to-many relationships apply pagination.
 
-## Many-to-Many
+## Many-to-One (BelongsTo)
+
+A many-to-one relationship has a object that references the foreign type.
+
+```graphql
+query {
+    animethemes {
+        data {
+            sequence
+            anime {
+                name
+            }
+        }
+    }
+}
+```
+
+## One-to-Many (HasMany)
+
+An one-to-many relationship has a list of objects that reference the related type.
+
+```graphql
+query {
+    animes {
+        data {
+            name
+            animethemes(first: 2) {
+                data {
+                    sequence
+                }
+            }
+        }
+    }
+}
+```
+
+## Many-to-Many (BelongsToMany)
 
 A many-to-many relationship has a `edges` list that contains edge objects.
 
@@ -65,4 +101,49 @@ will return the JSON:
         }
     }
 }
+```
+
+## Polymorphic Many-to-One (MorphTo)
+
+An union type indicates that a field might have multiple object types.
+
+```graphql
+query {
+    performances {
+        data {
+            artist {
+                ... on Artist {
+                    name
+                }
+                ... on Membership {
+                    group {
+                        name
+                    }
+                    member {
+                        name
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+## Polymorphic One-to-Many (MorphMany)
+
+The inverse relationship of polymorphic many-to-one. Applies pagination.
+
+```graphql
+query {
+    memberships {
+        data {
+            performances {
+                data {
+                    alias
+                    as
+                }
+            }
+        }
+    }
+}
 ```