Merge commit '9b0050e9aabe4be65c78ccf292a348f309d50ccd' as 'docs'

```
git subtree add --prefix=docs/ https://github.com/gohugoio/hugoDocs.git master --squash
```

Closes #11925

docs/content/en/functions/collections/IsSet.md

@@ -0,0 +1,45 @@
+---
+title: collections.IsSet
+description: Reports whether the key exists within the collection.
+categories: []
+keywords: []
+action:
+  aliases: [isset]
+  related:
+    - functions/go-template/if
+    - functions/go-template/with
+  returnType: bool
+  signatures: [collections.IsSet COLLECTION KEY]
+aliases: [/functions/isset]
+---
+
+For example, consider this site configuration:
+
+{{< code-toggle file=hugo >}}
+[params]
+showHeroImage = false
+{{< /code-toggle >}}
+
+It the value of `showHeroImage` is `true`, we can detect that it exists using either `if` or `with`:
+
+```go-html-template
+{{ if site.Params.showHeroImage }}
+  {{ site.Params.showHeroImage }} → true
+{{ end }}
+
+{{ with site.Params.showHeroImage }}
+  {{ . }} → true
+{{ end }}
+```
+
+But if the value of `showHeroImage` is `false`, we can't use either `if` or `with` to detect its existence. In this case, you must use the `isset` function:
+
+```go-html-template
+{{ if isset site.Params "showheroimage" }}
+  <p>The showHeroImage parameter is set to {{ site.Params.showHeroImage }}.<p>
+{{ end }}
+```
+
+{{% note %}}
+When using the `isset` function you must reference the key using lower case. See the previous example.
+{{% /note %}}