Clarify description of no-non-null-assertion (palantir#3172)

This commit clarifies that the `non-null-assertion` is the `!` postfix operator.
Even if the description was already correct, clearly stating what this rule is about `!` makes it easier to understand.

src/rules/noNonNullAssertionRule.ts

@@ -23,7 +23,7 @@ export class Rule extends Lint.Rules.AbstractRule {
     /* tslint:disable:object-literal-sort-keys */
     public static metadata: Lint.IRuleMetadata = {
         ruleName: "no-non-null-assertion",
-        description: "Disallows non-null assertions.",
+        description: "Disallows non-null assertions using the `!` postfix operator.",
         rationale: "Using non-null assertion cancels the benefits of the strict null checking mode.",
         optionsDescription: "Not configurable.",
         options: null,