docs(guide/directive): add note about HTML case-insensitivity

Closes angular#4719

Author: petebacondarwin

docs/content/guide/directive.ngdoc

@@ -56,9 +56,9 @@ The following also **matches** `ngModel`:
 Angular **normalizes** an element's tag and attribute name to determine which elements match which
 directives. We typically refer to directives by their case-sensitive
 {@link http://en.wikipedia.org/wiki/CamelCase camelCase} **normalized** name (e.g. `ngModel`). 
-However, in the DOM, we refer to directives by case-insensitive forms, typically using
-{@link http://en.wikipedia.org/wiki/Letter_case#Computers dash-delimited} attributes on DOM elements
-(e.g. `ng-model`).
+However, since HTML is case-insensitive, we refer to directives in the DOM by lower-case
+forms, typically using {@link http://en.wikipedia.org/wiki/Letter_case#Computers dash-delimited}
+attributes on DOM elements (e.g. `ng-model`).
 
 The **normalization** process is as follows: