Postprocessor only docs (#10899)

* add a note for only/except from cli to the post-processor template section

* typo; missing space

* Update website/content/docs/templates/hcl_templates/blocks/build/post-processor.mdx

Co-authored-by: Wilken Rivera <dev@wilkenrivera.com>

* tweak wording

Co-authored-by: Wilken Rivera <dev@wilkenrivera.com>

website/content/docs/templates/hcl_templates/blocks/build/post-processor.mdx

@@ -78,6 +78,21 @@ build {
 
 The values within `only` or `except` are _source names_, not builder types.
 
+If you want to exclude certain post-processors when running `packer build`
+from the command line, you can do so:
+
+`packer build --except=checksum mytemplate.pkr.hcl` will not run the checksum
+post-processor. This command line exclusion works by referencing post-processor
+_name_. By default, the post-processor is named after is its type, as
+demonstrated above in the `checksum` example. You can make a post-processor's
+name unique by adding a "name" field to each post-processor block.
+
+While the `-except` flag can be used to filter out post-processors on the
+command line, the `-only` flag does not work for post-processors. If you wish
+to only run a post-processor for a given source build  you must use the
+`only=[source]` syntax inside of your hcl templates, as described above.
+
+
 ## Build Contextual Variables
 
 Packer allows to access connection information and basic instance state

website/content/partials/commands/except.mdx

@@ -4,7 +4,7 @@
   `amazon-ebs` or `virtualbox-iso`, unless a specific `name` attribute is
   specified within the configuration. In HCL2 templates, the "name" is the
   source block's "name" label, unless an in-build source definition adds the
-  "name" configuration option.Any post-processor following
+  "name" configuration option. Any post-processor following
   a skipped post-processor will not run. Because post-processors can be nested
   in arrays a different post-processor chain can still run. A post-processor
   with an empty name will be ignored.