Add negation operator example to filter docs (#6150)

Author: weitzman

docs/output-formats-filters.md

@@ -108,6 +108,13 @@ Other fields in the output may be searched by using a simple expression in the `
 ```shell
 $ drush pm:list --filter='status=enabled'
 ```
+
+Exclude rows via the negation operator `!`:
+
+```shell
+drush watchdog:show --filter="type!=locale"
+```
+
 To search for fields that contain a string using the operator `*=`, or match a regular expression with the `~=` operator. For example, to find all views whose machine name contains the word *content*:
 ```shell
 drush views:list --filter='machine-name*=content'