[JENKINS-27474] Noting a few places where fileExists would be useful.

TUTORIAL.md

@@ -420,7 +420,7 @@ Here we are using the `def` keyword to define a function (you can also give a Ja
 
 The `readFile` step loads a text file from the workspace and returns its content.
 (Do _not_ try to use `java.io.File` methods, because these will refer to files on the master where Jenkins is running, not in the current workspace.)
-There is also a `writeFile` step to save content to a text file in the workspace.
+There is also a `writeFile` step to save content to a text file in the workspace, and a `fileExists` step to check whether a file exists without loading it.
 
 When you run the flow you should see
 

aggregator/src/test/java/org/jenkinsci/plugins/workflow/steps/scm/MercurialStepTest.java

@@ -90,7 +90,7 @@ private static void hg(File repo, String... cmds) throws Exception {
             "        }\n" +
             "        dir('other') {\n" +
             "            checkout([$class: 'MercurialSCM', source: $/" + otherRepoU + "/$, clean: true])\n" +
-            "            try {\n" +
+            "            try {\n" + // TODO or use fileExists
             "                readFile 'unversioned'\n" +
             "                error 'unversioned did exist'\n" +
             "            } catch (FileNotFoundException x) {\n" +

basic-steps/src/main/java/org/jenkinsci/plugins/workflow/steps/ReadFileStep.java

@@ -43,7 +43,7 @@ public final class ReadFileStep extends AbstractStepImpl {
         // Could be necessary in case a plugin installs a {@link LauncherDecorator} which keeps commands inside some kind of jail.
         // In that case we would need some API to determine that such a jail is in effect and this validation must be enforced.
         // But just checking the path is anyway not sufficient (due to crafted symlinks); would need to check the final resulting path.
-        // Same for WriteFileStep, PushdStep.
+        // Same for WriteFileStep, PushdStep, FileExistsStep.
         this.file = file;
     }