Add format descriptors infor to podman top

Trying to play with podman top, I had a hard time
finding info on format options.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

Closes: #1134
Approved by: umohnani8

cmd/podman/top.go

@@ -12,6 +12,16 @@ import (
 	"github.com/urfave/cli"
 )
 
+func getDescriptorString() string {
+	descriptors, err := libpod.GetContainerPidInformationDescriptors()
+	if err == nil {
+		return fmt.Sprintf(`
+Format Descriptors:
+%s`, strings.Join(descriptors, ","))
+	}
+	return ""
+}
+
 var (
 	topFlags = []cli.Flag{
 		LatestFlag,
@@ -20,11 +30,12 @@ var (
 			Hidden: true,
 		},
 	}
-	topDescription = `Display the running processes of the container.  Specify format descriptors
+	topDescription = fmt.Sprintf(`Display the running processes of the container.  Specify format descriptors
 to alter the output.  You may run "podman top -l pid pcpu seccomp" to print
 the process ID, the CPU percentage and the seccomp mode of each process of
 the latest container.
-`
+%s
+`, getDescriptorString())
 
 	topCommand = cli.Command{
 		Name:           "top",

docs/podman-top.1.md

@@ -24,6 +24,8 @@ to run containers such as CRI-O, the last started container could be from either
 
 The following descriptors are supported in addition to the AIX format descriptors mentioned in ps (1):
 
+**pcpu, group, ppid, user, args, comm, rgroup, nice, pid, pgid, etime, ruser, time, tty, vsz**
+
 **capinh**
 
   Set of inheritable capabilities. See capabilities (7) for more information.