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/migration/migration.c b/migration/migration.c
index 5df490a..9bd2ce7 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -768,7 +768,7 @@
     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;
     }