add hint when `options.md` is outdated (#5367)

Make it clearer what went wrong when lint fails due to 'options.md'
being out of date.

ci/Jenkinsfile

@@ -70,13 +70,16 @@ pipeline {
         sh 'make LOG_LEVEL=TRACE'
         /* Check documentation reflects `nimbus_beacon_node --help`. */
         sh '''#!/usr/bin/env bash
-          diff -u \\
+          if ! diff -u \\
               <(sed -n '/Usage/,/^...$/ { /^...$/d; p; }' \\
                 docs/the_nimbus_book/src/options.md) \\
               <(COLUMNS=200 build/nimbus_beacon_node --help | \\
                 sed -n '/Usage/,/Available sub-commands/ { /Available sub-commands/d; p; }' | \\
                 sed 's/\\x1B\\[[0-9;]*[mG]//g' | \\
-              sed 's/[[:space:]]*$//')
+                sed 's/[[:space:]]*$//'); then \\
+            echo "Please update 'options.md' to match 'nimbus_beacon_node --version'"; \\
+            false; \\
+          fi
         '''
       } }
     }