Document that IdentifierTrees may have null symbols in some cases to save the next person some time.

PiperOrigin-RevId: 735397059

Author: graememorgan

check_api/src/main/java/com/google/errorprone/util/ASTHelpers.java

@@ -259,6 +259,8 @@ public static boolean sameVariable(ExpressionTree expr1, ExpressionTree expr2) {
       return jcFieldAccess.sym;
     }
     if (tree instanceof JCIdent jcIdent) {
+      // You might reasonably expect that IdentifierTrees always have a non-null symbol, but a few
+      // cases don't, including module names and identifiers resolved from Javadoc (sometimes).
       return jcIdent.sym;
     }
     if (tree instanceof JCMethodInvocation jcMethodInvocation) {