[1.8.x] Fixed #23520 -- Explained that custom plural forms should be avoided

Thanks aruseni for the report.
Backport of 556eb67701 from master.

docs/topics/i18n/translation.txt

@@ -263,6 +263,15 @@ In a case like this, consider something like the following::
 
         a format specification for argument 'name', as in 'msgstr[0]', doesn't exist in 'msgid'
 
+.. note:: Plural form and po files
+
+    Django does not support custom plural equations in po files. As all
+    translation catalogs are merged, only the plural form for the main Django po
+    file (in ``django/conf/locale/<lang_code>/LC_MESSAGES/django.po``) is
+    considered. Plural forms in all other po files are ignored. Therefore, you
+    should not use different plural equations in your project or application po
+    files.
+
 .. _contextual-markers:
 
 Contextual markers