doc: Update docs/HANDLED_RULES.md (#925)

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docs/HANDLED_RULES.md

@@ -388,14 +388,15 @@ Example:
 #### Strings and Boxed types should be compared using "equals()" ([Sonar Rule 4973](https://rules.sonarsource.com/java/RSPEC-4973))
 
 Any comparison of strings or boxed types using `==` or `!=` is replaced by `equals`.
+By default, we use `equals` on the left operand, so we add a null check before comparison.
 
 Example:
 ```diff
 -        if (firstName == lastName) // Noncompliant
-+        if (firstName.equals(lastName))
++        if (firstName != null && firstName.equals(lastName))
 ...
 -        return b != a; // Noncompliant
-+        return !b.equals(a);
++        return b != null && !b.equals(a);
 ```
 
 Check out an accepted PR in [Apache Sling Discovery](https://github.com/apache/sling-org-apache-sling-discovery-impl/pull/1) that repairs one CompareStringsBoxedTypesWithEquals violation.