Update postprocess.md

Fixes #1080

content/en/hugo-pipes/postprocess.md

@@ -18,7 +18,7 @@ A prime use case for this is [CSS purging with PostCSS](#css-purging-with-postcs
 
 There are currently two limitations to this:
 
-1. This only works for `*.html` files.
+1. This only works in `*.html` templates (i.e. templates that produces HTML files).
 2. You cannot manipulate the values returned from the resource's methods. E.g. the `upper` in this example will not work as expected:
    
     ```go-html-template
@@ -67,4 +67,4 @@ Note that in the example above, the "CSS purge step" will only be applied to the
 {{ $css = $css | minify | fingerprint | resources.PostProcess }}
 {{ end }}
 <link href="{{ $css.RelPermalink }}" rel="stylesheet" />
-```
+```