Merge pull request #790 from enesonus/add-unknown-check

add unknown field check to validate command

Author: jbw976

content/master/cli/command-reference.md

@@ -828,7 +828,9 @@ Configuration/platform-ref-aws                             v0.9.0    True
 
 The `crossplane beta validate` command validates 
 [compositions]({{<ref "../concepts/compositions">}}) against provider or XRD 
-schemas using the Kubernetes API server's validation library.
+schemas using the Kubernetes API server's validation library
+with extra validation such as checking for unknown fields,
+a common source of difficult to debug issues in Crossplane.
 
 The `crossplane beta validate` command supports validating the following 
 scenarios: