Add a tip about the default emojify catalog

Author: javiereguiluz

string.rst

@@ -669,6 +669,11 @@ all services:
     {{ 'I like :kiwi:'|emojify('text') }}       {# renders: I like 🥝 #}
     {{ 'I like :kiwifruit:'|emojify('text') }}  {# renders: I like 🥝 #}
 
+.. tip::
+
+    The ``text`` catalog is used by default when no catalog is passed, so calling
+    ``|emojify('text')`` is the same as calling ``|emojify``.
+
 Removing Emojis
 ~~~~~~~~~~~~~~~