Document -on-error on the "packer build" page

command/build.go

@@ -288,9 +288,9 @@ Options:
   -except=foo,bar,baz        Build all builds other than these
   -force                     Force a build to continue if artifacts exist, deletes existing artifacts
   -machine-readable          Machine-readable output
-  -on-error=abort            When a builder fails, abort without cleanup
-  -on-error=ask              When a builder fails, prompt for action
-  -on-error=cleanup          When a builder fails, clean up and exit (the default)
+  -on-error=abort            If the build fails, abort without cleanup
+  -on-error=ask              If the build fails, prompt for action
+  -on-error=cleanup          If the build fails, clean up and exit (the default)
   -only=foo,bar,baz          Only build the given builds by name
   -parallel=false            Disable parallelization (on by default)
   -var 'key=value'           Variable for templates, can be used multiple times.

website/source/docs/command-line/build.html.md

@@ -36,6 +36,13 @@ artifacts that are created will be outputted at the end of the build.
     remove the artifacts from the previous build. This will allow the user to
     repeat a build without having to manually clean these artifacts beforehand.
 
+-   `-on-error=cleanup` (default), `-on-error=abort`, `-on-error=ask` - Selects
+    what to do when the build fails.  `cleanup` cleans up after the previous
+    steps, deleting temporary files and virtual machines.  `abort` exits without
+    any cleanup, but the next build may require `-force`.  `ask` presents a
+    prompt and waits for you to decide to clean up, abort, or retry the failed
+    step.
+
 -   `-only=foo,bar,baz` - Only build the builds with the given
     comma-separated names. Build names by default are the names of their
     builders, unless a specific `name` attribute is specified within