migrate-start-postcopy: Improve text

Improve the text in both the qapi-schema and hmp help to point out
you need to set the postcopy-ram capability prior to issuing
migrate-start-postcopy.

Also fix the text of the migrate_start_postcopy error that
deals with capabilities.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Acked-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index f3de407..bb52e4d 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -1025,7 +1025,9 @@
         .name       = "migrate_start_postcopy",
         .args_type  = "",
         .params     = "",
-        .help       = "Switch migration to postcopy mode",
+        .help       = "Followup to a migration command to switch the migration"
+                      " to postcopy mode. The x-postcopy-ram capability must "
+                      "be set before the original migration command.",
         .mhandler.cmd = hmp_migrate_start_postcopy,
     },