Add comments explaining when unexpected specializations may be run.

Author: aardvark179

src/main/java/org/truffleruby/cext/CExtNodes.java

@@ -1402,7 +1402,8 @@ public DynamicObject addToMarkList(VirtualFrame frmae, TruffleObject guardedObje
 
         @Fallback
         public DynamicObject addToMarkList(VirtualFrame frmae, Object guardedObject) {
-            // Do nothing for unexpected objects, no matter how unexpected.
+            // Do nothing for unexpected objects, no matter how unexpected. This can occur inside
+            // macros that guard a variable which may not have been initialized.
             return nil();
         }
 

src/main/java/org/truffleruby/cext/UnwrapNode.java

@@ -73,6 +73,8 @@ public Object unwrapTaggedObject(long handle) {
 
         @Fallback
         public ValueWrapper unWrapUnexpectedHandle(long handle) {
+            // Avoid throwing a specialization exception when given an uninitialized or corrupt
+            // handle.
             return null;
         }
 
@@ -118,6 +120,8 @@ public ValueWrapper unwrapTaggedObject(long handle) {
 
         @Fallback
         public ValueWrapper unWrapUnexpectedHandle(long handle) {
+            // Avoid throwing a specialization exception when given an uninitialized or corrupt
+            // handle.
             return null;
         }