Modify kubectl exec help to set -c CONTAINER as optional

docs/man/man1/kubectl-exec.1

@@ -19,7 +19,7 @@ Execute a command in a container.
 .SH OPTIONS
 .PP
 \fB\-c\fP, \fB\-\-container\fP=""
-    Container name
+    Container name. If omitted, the first container in the pod will be chosen
 
 .PP
 \fB\-h\fP, \fB\-\-help\fP=false

docs/user-guide/kubectl/kubectl_exec.md

@@ -41,7 +41,7 @@ Execute a command in a container.
 Execute a command in a container.
 
 ```
-kubectl exec POD -c CONTAINER -- COMMAND [args...]
+kubectl exec POD [-c CONTAINER] -- COMMAND [args...]
 ```
 
 ### Examples
@@ -61,7 +61,7 @@ $ kubectl exec 123456-7890 -c ruby-container -i -t -- bash -il
 ### Options
 
 ```
-  -c, --container="": Container name
+  -c, --container="": Container name. If omitted, the first container in the pod will be chosen
   -h, --help[=false]: help for exec
   -p, --pod="": Pod name
   -i, --stdin[=false]: Pass stdin to the container