Document util.set_lang_class (see #1737)

website/api/_top-level/_util.jade

@@ -51,7 +51,9 @@ p
 p
     |  Import and load a #[code Language] class. Allows lazy-loading
     |  #[+a("/usage/adding-languages") language data] and importing
-    |  languages using the two-letter language code.
+    |  languages using the two-letter language code. To add a language code
+    |  for a custom language class, you can use the
+    |  #[+api("top-level#util.set_lang_class") #[code set_lang_class]] helper.
 
 +aside-code("Example").
     for lang_id in ['en', 'de']:
@@ -70,6 +72,33 @@ p
         +cell #[code Language]
         +cell Language class.
 
++h(3, "util.set_lang_class") util.set_lang_class
+    +tag function
+
+p
+    |  Set a custom #[code Language] class name that can be loaded via
+    |  #[+api("top-level#util.get_lang_class") #[code get_lang_class]]. If
+    |  your model uses a custom language, this is required so that spaCy can
+    |  load the correct class from the two-letter language code.
+
++aside-code("Example").
+    from spacy.lang.xy import CustomLanguage
+
+    util.set_lang_class('xy', CustomLanguage)
+    lang_class = util.get_lang_class('xy')
+    nlp = lang_class()
+
++table(["Name", "Type", "Description"])
+    +row
+        +cell #[code name]
+        +cell unicode
+        +cell Two-letter language code, e.g. #[code 'en'].
+
+    +row
+        +cell #[code cls]
+        +cell #[code Language]
+        +cell The language class, e.g. #[code English].
+
 +h(3, "util.load_model") util.load_model
     +tag function
     +tag-new(2)