Add clarification on how to enable path matching

The fact that you need to explicitly add the annotation is easy to miss.
This makes this more explicit, while leaving the finer details to the
linked annotations document.

docs/user-guide/ingress-path-matching.md

@@ -6,6 +6,7 @@
     Regular expressions and wild cards are not supported in the `spec.rules.host` field. Full hostnames must be used. 
 
 The ingress controller supports **case insensitive** regular expressions in the `spec.rules.http.paths.path` field.
+This can be enabled by setting the `nginx.ingress.kubernetes.io/use-regex` annotation to `true` (the default is false).
 
 See the [description](./nginx-configuration/annotations.md#use-regex) of the `use-regex` annotation for more details.