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>

hmp-commands.hx

@@ -1025,7 +1025,9 @@ ETEXI
         .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,
     },
 

migration/migration.c

@@ -768,7 +768,7 @@ void qmp_migrate_start_postcopy(Error **errp)
     MigrationState *s = migrate_get_current();
 
     if (!migrate_postcopy_ram()) {
-        error_setg(errp, "Enable postcopy with migration_set_capability before"
+        error_setg(errp, "Enable postcopy with migrate_set_capability before"
                          " the start of migration");
         return;
     }

qapi-schema.json

@@ -706,7 +706,9 @@
 ##
 # @migrate-start-postcopy
 #
-# Switch migration to postcopy mode
+# 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.
 #
 # Since: 2.5
 { 'command': 'migrate-start-postcopy' }