docs: Add instructions on buildExpired in build configurations

docs/content/overview/configuration.md

@@ -87,6 +87,8 @@ Following is a list of Hugo-defined variables that you can configure and their c
     buildDrafts:                false
     # include content with publishdate in the future
     buildFuture:                false
+    # include content already expired
+    buildExpired:               false
     # enable this to make all relative URLs relative to content root. Note that this does not affect absolute URLs.
     relativeURLs:               false
     canonifyURLs:               false