added clarification about limit modifier

Closes ExpressionEngine/ExpressionEngine#3811

Author: intoeetive

docs/templates/variable-modifiers.md

@@ -135,6 +135,8 @@ Outputs the length of the content in characters.
 
 Limits the content to the specified number of characters. Without `preserve_words='n'` may be fewer than the exact limit, as this retains whole words.
 
+Note that all HTML formatting will be stripped out automatically before applying limit modifier.
+
     {excerpt:limit characters='20'}
     {!-- A discussion… --}