Update waf-front-door-custom-rules.md

clarifying that size is measured in bytes.

Author: Miskatonic-Electronic

articles/web-application-firewall/afds/waf-front-door-custom-rules.md

@@ -169,7 +169,7 @@ Here's an example JSON description of the custom rule:
 
 ### Size constraint
 
-Front Door's WAF enables you to build custom rules that apply a length or size constraint on a part of an incoming request.
+Front Door's WAF enables you to build custom rules that apply a length or size constraint on a part of an incoming request. This size constraint is measured in bytes.
 
 Suppose you need to block requests where the URL is longer than 100 characters.