Update docs/pipelines.md

Co-authored-by: Jerop Kipruto <[email protected]>

docs/pipelines.md

@@ -782,7 +782,7 @@ There are a lot of scenarios where `when` expressions can be really useful. Some
 
 CEL (Common Expression Language) is a declarative language designed for simplicity, speed, safety, and portability which can be used to express a wide variety of conditions and computations.
 
-You can define a CEL expression in `WhenExpression` to guard the execution of a `Task`. You can use a single line of CEL string to replace current `WhenExpressions`'s `input`+`operator`+`values`. For example:
+You can define a CEL expression in `WhenExpression` to guard the execution of a `Task`.  The CEL expression must evaluate to either `true` or `false`. You can use a single line of CEL string to replace current `WhenExpressions`'s `input`+`operator`+`values`. For example:
 
 ```yaml
 # current WhenExpressions