docs: Update to show undefined example with !=

Fixes #1626

Signed-off-by: Torin Sandall <torinsandall@gmail.com>

docs/content/how-do-i-write-policies.md

@@ -83,22 +83,27 @@ You can define a new concept using a rule.  For example, `v` below is true if th
 v { "hello" == "world" }
 ```
 
-If we evaluate `v`, the result is `undefined` because the body
-of the rule never evaluates to `true`. As a result, the document generated by
-the rule is not defined.
+If we evaluate `v`, the result is `undefined` because the body of the rule never
+evaluates to `true`. As a result, the document generated by the rule is not
+defined.
 
 ```live:eg/undefined:query:hidden
 v
 ```
 ```live:eg/undefined:output:expect_undefined
 ```
 
-Expressions that refer to undefined values are also undefined.
+Expressions that refer to undefined values are also undefine. This includes comparisons such as `!=`.
 
 ```live:eg/undefined/expression:query:merge_down
 v == true
 ```
-```live:eg/undefined/expression:output:expect_undefined
+```live:eg/undefined/expression:output:expect_undefined,merge_down
+```
+```live:eg/undefined/other_expression:query:merge_down
+v != true
+```
+```live:eg/undefined/other_expression:output:expect_undefined
 ```
 
 We can define rules in terms of [Variables](#variables) as well:
@@ -181,7 +186,7 @@ sites = [{"name": "prod"}, {"name": "smoke1"}, {"name": "dev"}]
 And
 ```live:eg/references/basic:module
 r { sites[_].name == "prod" }
-``` 
+```
 
 The rule `r` above asserts that there exists (at least) one document within `sites` where the `name` attribute equals `"prod"`.