Fix misleading reason for uncovered arg deprecation case

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: wagenet

docs/rules/template-no-deprecated.md

@@ -19,7 +19,7 @@ This rule checks if imported Glimmer components, helpers, or modifiers are marke
 | Block component         | `{{#DeprecatedBlock}}…{{/DeprecatedBlock}}` |
 | Modifier                | `<div {{deprecatedModifier}}>`              |
 
-**Not covered:** `<MyComp @deprecatedArg={{x}}>` — argument names are not scope-registered by the parser.
+**Not covered:** `<MyComp @deprecatedArg={{x}}>` — checking argument deprecations is not yet implemented.
 
 ## Examples